Use strings for class_name

As mentioned in the Rails console:

DEPRECATION WARNING: Passing a class to the `class_name` is deprecated
and will raise an ArgumentError in Rails 5.2. It eagerloads more classes
than necessary and potentially creates circular dependencies. Please
pass the class name as a string.

app/models/budget.rb

@@ -34,7 +34,7 @@ class Budget < ApplicationRecord
   has_many :groups, dependent: :destroy
   has_many :headings, through: :groups
   has_many :lines, through: :ballots, class_name: "Budget::Ballot::Line"
-  has_many :phases, class_name: Budget::Phase
+  has_many :phases, class_name: "Budget::Phase"
 
   has_one :poll