Add project setting to force usage of squash merge

Description

We love the new squash merge feature in GitLab and have updated our team's process documentation to tell devs to use it on every MR. However, they need to remember to check the box on every MR. It would be great if we could enforce this at the project level instead of relying on devs checking it on each and every MR.

Proposal

Add a checkbox to the project settings page to "Require squashing of commits on all merge requests". If that project setting is checked, then the "Squash commits" checkbox is automatically checked and disabled on all MRs for the project.