Improve text of MR merge widget and manual merge modal

Modal with manual merge instructions:

Archived:

WIP:

Missing target branch:

Missing source branch:

Nothing to merge:

Checking ability to merge:

Conflicts when allowed to merge:

Conflicts when not allowed to merge:

Mergeable when allowed to merge:

Mergeable when not allowed to merge:

Merge in progress:

Merge failed:

Merged:

Closed:

cc @dzaporozhets @rspeicher