Enable editing squash settings for All branches

What does this MR do and why?

1. Enables editing when target is All branches
2. Also fixes a small bug where users could not save their selection when no default squash setting was set yet.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the branch_rule_squash_settings feature flag.
2. Create a project in a group.
3. Navigate to Project > Settings > Repository.
4. Click on View details next to All branches.
5. On the Branch rule details page, the Squash commits when merging section should have an Edit button.
6. Clicking the edit button should open a drawer and editing should work as expected (both EE and CE).

Related to #512674 (closed)