Fix crash destroying budgets with admins/valuators

We don't allow deleting a budget with associated investments. However,
we allow deleting a budget with associated administrators and valuators.
This results in a foreign key violation error:

PG::ForeignKeyViolation: ERROR:  update or delete on table "budgets"
violates foreign key constraint "fk_rails_c847a52b1d" on table
"budget_administrators"

Using the `dependent: :destroy` option when defining the relationship,
we remove the association records when removing the budget.

As a bonus, we reduce the number of Rubocop offenses regarding the
`Rails/HasManyOrHasOneDependent` rule. Only 72 to go! :)

app/models/budget.rb

@@ -34,9 +34,9 @@ class Budget < ApplicationRecord
   has_many :headings, through: :groups
   has_many :lines, through: :ballots, class_name: "Budget::Ballot::Line"
   has_many :phases, class_name: "Budget::Phase"
-  has_many :budget_administrators
+  has_many :budget_administrators, dependent: :destroy
   has_many :administrators, through: :budget_administrators
-  has_many :budget_valuators
+  has_many :budget_valuators, dependent: :destroy
   has_many :valuators, through: :budget_valuators
 
   has_one :poll