WIP: Squash options

What does this MR do?

This merge request adds configuring defaults for squashing on a project-level: #17613 (closed)

There are four aspects to this:

• Change the Project model to have squash options "always", "never", "enabled", or "disabled"
• Allow the Project model squash options to be set in the repository's settings
• The above squash options should effect creating a new merge request
• The above squash options should effect currently existing merge requests by failing them if they submit with an invalid option

Screenshots

Settings -> General -> Merge requests

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

The primary thing to test would be how do pre-existing PRs function after this option is changed on the project-level? Tests need to ensure those edge cases are fine. This means the squash options needs to be enforced on a model-level.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done