Move proposals form partial to a component

Since now categories are loaded in both the investment form component
and proposal form component, and their controllers are the only
"commentable" controllers using the `@categories` instance variable, we
can remove the `load_categories` call in `CommentableActions` affecting
the create and update actions.

app/components/proposals/form_component.rb

@@ -0,0 +1,17 @@
+class Proposals::FormComponent < ApplicationComponent
+  include TranslatableFormHelper
+  include GlobalizeHelper
+  attr_reader :proposal, :url
+  delegate :current_user, :suggest_data, :geozone_select_options, to: :helpers
+
+  def initialize(proposal, url:)
+    @proposal = proposal
+    @url = url
+  end
+
+  private
+
+    def categories
+      Tag.category.order(:name)
+    end
+end