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
 

app/models/community.rb

@@ -1,6 +1,6 @@
 class Community < ApplicationRecord
   has_one :proposal
-  has_one :investment, class_name: Budget::Investment
+  has_one :investment, class_name: "Budget::Investment"
   has_many :topics
 
   def participants

app/models/map_location.rb

@@ -1,7 +1,7 @@
 class MapLocation < ApplicationRecord
 
   belongs_to :proposal, touch: true
-  belongs_to :investment, class_name: Budget::Investment, touch: true
+  belongs_to :investment, class_name: "Budget::Investment", touch: true
 
   validates :longitude, :latitude, :zoom, presence: true, numericality: true
 

app/models/poll/ballot.rb

@@ -1,5 +1,5 @@
 class Poll::Ballot < ApplicationRecord
-  belongs_to :ballot_sheet, class_name: Poll::BallotSheet
+  belongs_to :ballot_sheet, class_name: "Poll::BallotSheet"
 
   validates :ballot_sheet_id, presence: true
 

app/models/poll/ballot_sheet.rb

@@ -1,7 +1,7 @@
 class Poll::BallotSheet < ApplicationRecord
   belongs_to :poll
   belongs_to :officer_assignment
-  has_many :ballots, class_name: Poll::Ballot
+  has_many :ballots, class_name: "Poll::Ballot"
 
   validates :data, presence: true
   validates :poll_id, presence: true