[meta] Resolve merge conflicts in the web interface

Steps:

Proposal of functionality (below)
Mockup of the design
Backend
Frontend

Proposal

In two iterations:

You are able to choose theirs and ours for each conflict
You can edit inline to resolve conflicts completely

old issue

This would be a game changer for using git via the browser.

The feature request is at http://feedback.gitlab.com/forums/176466-general/suggestions/5590496-resolve-any-merge-request-conflict-from-the-web-in

A proposal is at https://github.com/gitlabhq/gitlabhq/pull/7345

/cc @dzaporozhets @JobV

Zendesk issue: https://gitlab.zendesk.com/agent/tickets/12278