Add option to perform a standard fast-forward merge when accepting merge requests.

Adds a new checkbox next to the "Remove source-branch" checkbox that when checked will perform a standard fast-forward merge when accepting the merge request. A new config entry is also added allowing to set the default state of the checkbox.
By default, target original repository if fork when creating new MR.
Add button for removing source branch for merged MR's.
Always link to comments for merge request notes.

This will avoid issues with updated merge requests where the comments are outdated.