avoid publishing empty posts (fixes #4401)

invalidate empty posts when created by a local user

add changelog entry
Merge pull request #4542 from Raven24/empty_publishing

avoid publishing empty posts
refactor ConversationsController, move query builing to User model
Merge pull request #4547 from Raven24/conversations

refactor conversations_controller a bit
refactor twitter service model
Merge pull request #4387 from MrZYX/refactor_twitter

refactor twitter service model
refactor conversations_controller#create
Merge pull request #4551 from Raven24/conversations

refactor conversations_controller#create
refactor message creation (conversations)
Merge pull request #4556 from Raven24/conversations

refactor conversation message creation