External Merge Request Integration

Description

In the same way as we have an External Issue Tracker and external Wiki Integration it would be nice to allow for external merge request integration. This makes it nicer to integrate gitlab with an external Pull Request system such as Phabricator's Differential.

Proposal

It should work very similarly to External Wiki Integration. You just provide a link and Merge Requests will link to that. In addition it probably would be nice to also be able to customize the text that gitlab shows when you push a new branch.

Links / references

Documentation blurb

(Write the start of the documentation of this feature here, include:

1. We want to make integration with an external Pull request system neater.
2. Provide a new External Merge Request Integration
3. In the same manner as the External Wiki Integraton is used.

During implementation, this can then be copied and used as a starter for the documentation.)