[Draft] Commit changes modal: Disable commit to default branch for locked file/directory

Problem

Context: #516868 (closed)

Our implementation of File Locking is representative of soft locks. Users are not able to merge changes into the default branch if a file/directory is locked. Users on non-default branches can still make changes to the locked file/directory on non-default branches. This impacts the following actions on the Single File page:

• Replace
• Delete
• Edit > Commit changes

When a file is locked by another user, the Unlock action is not available to other users on the default branch, and non-default branches. The Replace and Delete actions, and Commit changes radio button to commit changes to the default branch are still available to users on the default branch, even though file modification is restricted to the user who has permission to unlock the file or Maintainers.

It's possible to go through replacing and deleting flow, which eventually returns an error from the backend when submitting the change. This is not user-friendly.

Implementation breakdown

Design change	Issue
Default branch updates Default branch Disable the Replace and Delete buttons for users who do not have permission to unlock a file	#516868 (closed)
Commit changes modal updates Non-default branches Update the commit changes modal in the Replace Delete Edit flows to inform users of locking status when they do not have permission to unlock a file	#521439 (this)
Edit page updates Update the Edit page or related flows to inform users of locking status when they do not have permission to unlock a file	#521831

Design Proposal

We should indicate the user that they cannot perform the action earlier and prevent them into running to an error. The following changes should be addressed for the commit changes modal for the Replace Delete Edit actions on the single file page.

Replace

	User HAS permission to commit to branch + HAS permission to unlock file	User HAS permission to commit to branch + does NOT have permission to unlock file	User does NOT have permission to commit to branch + does NOT have permission to unlock file
Default branch	No change	TBD	TBD
Non-default branch	No change	TBD	TBD

Delete

	User HAS permission to commit to branch + HAS permission to unlock file	User HAS permission to commit to branch + does NOT have permission to unlock file	User does NOT have permission to commit to branch + does NOT have permission to unlock file
Default branch	No change	TBD	TBD
Non-default branch	No change	TBD	TBD

Edit

	User HAS permission to commit to branch + HAS permission to unlock file	User HAS permission to commit to branch + does NOT have permission to unlock file	User does NOT have permission to commit to branch + does NOT have permission to unlock file
Default branch	No change	You don't have permission to modify this locked file on main. What is file locking?	You don't have permission to modify this locked file or commit to main. What are protected branches and file locking?
Non-default branch	No change	TBD	TBD

TBD @alyssatrinh