Remove participants limit for conversations

The limit was added in 2012 to prevent spam, but since the participants
need to be a mutual contact with the author nowadays, I don't think it's
a spam problem anymore.

app/models/conversation.rb

@@ -9,13 +9,8 @@ class Conversation < ApplicationRecord
   has_many :participants, class_name: "Person", through: :conversation_visibilities, source: :person
   has_many :messages, -> { order("created_at ASC") }, inverse_of: :conversation
 
-  validate :max_participants
   validate :local_recipients
 
-  def max_participants
-    errors.add(:max_participants, "too many participants") if participants.count > 20
-  end
-
   def local_recipients
     recipients.each do |recipient|
       if recipient.local?