Improve Merge Request accept flow with automatic rebase and option to squash

We have different Merge Request merge strategies:

1. Merge commit
2. Merge commit with semi-linear history
3. Fast-forward merge

Two things:

1. The last two ones can be improved a lot by automatically rebase before merging, instead of requiring a manual button click: https://gitlab.com/gitlab-org/gitlab-ee/issues/408

2. Besides that, we wan to add a checkbox to squash all of this MRs commits into one commit before merging https://gitlab.com/gitlab-org/gitlab-ee/issues/150

1 requires a bit of refactoring and rethinking of the flow, and 2 will be much easier once 1 has happened.

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Edit:

This issue has been split into two separate issues: #895 and #896 (closed) (https://gitlab.com/gitlab-org/gitlab-ee/issues/150)