Remove duplication in code to validate video URL

We were using the same code, and the same regular expressions, in two
places. To do so, we were including a helper inside a model, which is
something we don't usually do.

app/models/proposal.rb

@@ -14,7 +14,7 @@ class Proposal < ApplicationRecord
   include Mappable
   include Notifiable
   include Documentable
-  include EmbedVideosHelper
+  include Videoable
   include Relationable
   include Milestoneable
   include Randomizable
@@ -59,8 +59,6 @@ class Proposal < ApplicationRecord
 
   validates :terms_of_service, acceptance: { allow_nil: false }, on: :create
 
-  validate :valid_video_url?
-
   before_validation :set_responsible_name
 
   before_save :calculate_hot_score, :calculate_confidence_score