Proposal: make squash-merging the default behavior for gitlab MRs

[David Hurka]

> Even better might be to force an explicit decision by not having a default
> for this at all, e.g. by offering "Rebase" and "Squash + Rebase" actions
> next to each other.

The squash checkbox is available directly next to the Rebase/Merge button; 
provided the “Allow Contributions” checkbox was checked when creating the MR.