Add Rails/AddColumnIndex rubocop rule

The `column` method in ActiveRecord::ConnectionAdapters::TableDefinition
supports adding the `index:` option. The documentation says:

> Instantiates a new column for the table. See connection.add_column for
> available options.
>
> Additional options are:
>
> :index - Create an index for the column. Can be either true or an
> options hash.

So basically the `connection.add_column` method silently ignores the
`index:` option, and whenever we intended to create an index this way,
we didn't.

We're creating a new migration where we properly add the indexes that
weren't added when we intended to.

Thanks to the rubocop-rails team, who added this cop in version 2.11.0
and helped us notice this bug.

db/migrate/20151028145921_add_confidence_score_to_comments.rb

@@ -1,5 +1,5 @@
 class AddConfidenceScoreToComments < ActiveRecord::Migration[4.2]
   def change
-    add_column :comments, :confidence_score, :integer, index: true
+    add_column :comments, :confidence_score, :integer
   end
 end

db/migrate/20160411161531_add_genre_and_dob_to_users.rb

@@ -1,6 +1,6 @@
 class AddGenreAndDobToUsers < ActiveRecord::Migration[4.2]
   def change
-    add_column :users, :genre, :string, index: true, limit: 10
-    add_column :users, :date_of_birth, :datetime, index: true
+    add_column :users, :genre, :string, limit: 10
+    add_column :users, :date_of_birth, :datetime
   end
 end

db/migrate/20160610094658_desnormalize_ballot_line.rb

@@ -1,7 +1,7 @@
 class DesnormalizeBallotLine < ActiveRecord::Migration[4.2]
   def change
-    add_column :budget_ballot_lines, :budget_id, :integer, index: true
-    add_column :budget_ballot_lines, :group_id, :integer, index: true
-    add_column :budget_ballot_lines, :heading_id, :integer, index: true
+    add_column :budget_ballot_lines, :budget_id, :integer
+    add_column :budget_ballot_lines, :group_id, :integer
+    add_column :budget_ballot_lines, :heading_id, :integer
   end
 end

db/migrate/20160905092539_denormalize_investments.rb

@@ -1,6 +1,6 @@
 class DenormalizeInvestments < ActiveRecord::Migration[4.2]
   def change
-    add_column :budget_investments, :budget_id, :integer, index: true
-    add_column :budget_investments, :group_id, :integer, index: true
+    add_column :budget_investments, :budget_id, :integer
+    add_column :budget_investments, :group_id, :integer
   end
 end

db/migrate/20161221172447_add_selected_to_budget_investment.rb

@@ -1,5 +1,5 @@
 class AddSelectedToBudgetInvestment < ActiveRecord::Migration[4.2]
   def change
-    add_column :budget_investments, :selected, :bool, default: false, index: true
+    add_column :budget_investments, :selected, :bool, default: false
   end
 end

db/migrate/20170120161058_add_geozone_restricted_to_polls.rb

@@ -1,5 +1,5 @@
 class AddGeozoneRestrictedToPolls < ActiveRecord::Migration[4.2]
   def change
-    add_column :polls, :geozone_restricted, :boolean, default: false, index: true
+    add_column :polls, :geozone_restricted, :boolean, default: false
   end
 end

db/migrate/20170208110146_add_officer_id_to_failed_census_calls.rb

@@ -1,6 +1,6 @@
 class AddOfficerIdToFailedCensusCalls < ActiveRecord::Migration[4.2]
   def change
-    add_column :failed_census_calls, :poll_officer_id, :integer, index: true
+    add_column :failed_census_calls, :poll_officer_id, :integer
     add_foreign_key :failed_census_calls, :poll_officers
   end
 end

db/migrate/20170703120055_add_incompatible_to_budget_investments.rb

@@ -1,5 +1,5 @@
 class AddIncompatibleToBudgetInvestments < ActiveRecord::Migration[4.2]
   def change
-    add_column :budget_investments, :incompatible, :bool, default: false, index: true
+    add_column :budget_investments, :incompatible, :bool, default: false
   end
 end

db/migrate/20190410132842_add_selected_to_proposal.rb

@@ -1,5 +1,5 @@
 class AddSelectedToProposal < ActiveRecord::Migration[4.2]
   def change
-    add_column :proposals, :selected, :bool, default: false, index: true
+    add_column :proposals, :selected, :bool, default: false
   end
 end

db/migrate/20210811195800_add_missing_indexes.rb

@@ -0,0 +1,17 @@
+class AddMissingIndexes < ActiveRecord::Migration[5.2]
+  def change
+    add_index :comments, :confidence_score
+    add_index :users, :gender
+    add_index :users, :date_of_birth
+    add_index :budget_ballot_lines, :budget_id
+    add_index :budget_ballot_lines, :group_id
+    add_index :budget_ballot_lines, :heading_id
+    add_index :budget_investments, :budget_id
+    add_index :budget_investments, :group_id
+    add_index :budget_investments, :selected
+    add_index :polls, :geozone_restricted
+    add_index :failed_census_calls, :poll_officer_id
+    add_index :budget_investments, :incompatible
+    add_index :proposals, :selected
+  end
+end