Squash commits when merging a Merge request

Closing this issue in favor of #150 (closed). Please participate there if any other comments.

Description

This issue has to be done after #895 and is part of #411 (closed) .

The idea is to squash commits together into one before merging a merge request.

Specifications

• Add an option in the Project settings to allow this functionality
• Add a dropdown on the Merge Request screen with the option to squash commits
• When selected, an input field and a textarea appear to let the user enter the squash commit message.

Wireframes

Option in the Settings panel of a project:

Flow in the Merge Request screen

Reference

• Part of issue #411 (closed) (Improve Merge Request accept flow with automatic rebase and option to squash)
• Related to #150 (closed)
• Should be done after #895 (Simplification of the merging process through the UI)

cc/ @JobV @DouweM