Merge branch 'lock_for_issuable' into 'master'

Optimistic locking for Issue and Merge Requests

## What does this MR do?

It implements Optimistic Locking http://api.rubyonrails.org/classes/ActiveRecord/Locking/Optimistic.html for Issues and MRs but in a bit custom way. We use it for cases when either title or description field is involved in the change.

## Why was this MR needed?

We have to prevent data losing.

## What are the relevant issue numbers?
https://gitlab.com/gitlab-org/gitlab-ce/issues/12940

## Screenshots (if relevant)

![Screen_Shot_2016-07-09_at_15.33.02](/uploads/2efbe547a12fda93bce347fa711438d7/Screen_Shot_2016-07-09_at_15.33.02.png)


See merge request !5146