Set default value for merge request options

Description

It's quite usual to see colleagues forgetting to check "Remove source branch when merge request is accepted." when creating merge requests. I'd expect the same to happen with the new option "Squash commits when merge request is accepted.".

Proposal

The solution for this may be quite simple:

• Add a small form in the project settings page to let users define a default value for these two options.
• In order to generate no side-effect, the default value continues to be 'false' and people with proper permission may change it to be 'true', so that any new merge request gets both options checked by default.