Add valuation boolean flag to Comment model

Why:

Budget Investment's valuators need to be able to comment on investments
without making those comments public. We need a way to clearly make a
distinction to avoid "leaking" internal valuation comments.

How:

Adding a boolean `valuation` attribute defaulted to false to the Comments
table, and index on it with concurrent algorithm as explained at
https://robots.thoughtbot.com/how-to-create-postgres-indexes-concurrently-in

The name `valuation` was chosen instead of `internal` because of the more
specific meaning as well as avoiding a collision with existing internal_comments
attribute on Budget::Investment model (soon to be deprecated & removed)

db/migrate/20180129190931_add_valuation_flag_to_comments.rb

@@ -0,0 +1,5 @@
+class AddValuationFlagToComments < ActiveRecord::Migration
+  def change
+    add_column :comments, :valuation, :boolean, default: false
+  end
+end

db/migrate/20180129190950_add_index_to_valuation_comments.rb

@@ -0,0 +1,7 @@
+class AddIndexToValuationComments < ActiveRecord::Migration
+  disable_ddl_transaction!
+
+  def change
+    add_index :comments, :valuation, algorithm: :concurrently
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20180119073228) do
+ActiveRecord::Schema.define(version: 20180129190950) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -234,7 +234,7 @@ ActiveRecord::Schema.define(version: 20180119073228) do
     t.string   "commentable_type"
     t.text     "body"
     t.string   "subject"
-    t.integer  "user_id",                        null: false
+    t.integer  "user_id",                            null: false
     t.datetime "created_at"
     t.datetime "updated_at"
     t.datetime "hidden_at"
@@ -247,7 +247,8 @@ ActiveRecord::Schema.define(version: 20180119073228) do
     t.integer  "cached_votes_down",  default: 0
     t.datetime "confirmed_hide_at"
     t.string   "ancestry"
-    t.integer  "confidence_score",   default: 0, null: false
+    t.integer  "confidence_score",   default: 0,     null: false
+    t.boolean  "valuation",          default: false
   end
 
   add_index "comments", ["ancestry"], name: "index_comments_on_ancestry", using: :btree
@@ -257,6 +258,7 @@ ActiveRecord::Schema.define(version: 20180119073228) do
   add_index "comments", ["commentable_id", "commentable_type"], name: "index_comments_on_commentable_id_and_commentable_type", using: :btree
   add_index "comments", ["hidden_at"], name: "index_comments_on_hidden_at", using: :btree
   add_index "comments", ["user_id"], name: "index_comments_on_user_id", using: :btree
+  add_index "comments", ["valuation"], name: "index_comments_on_valuation", using: :btree
 
   create_table "communities", force: :cascade do |t|
     t.datetime "created_at", null: false