Edit/Delete file: Move commit flow of single file editor into a modal

ℹ️ For latest discussion see: #444302 (comment 2192732639) and !170512 (merged)

Proposal

Based on non-confidential info from https://gitlab.com/groups/gitlab-org/-/epics/9933+

When deleting a file or updating a file it results in a commit, so we can consolidate the flow into a "Commit changes" modal.

🎨 Initial design thinking was done in epic 9933#designs. After discussion and #444302 (comment 2158381424), the designs have been updated below. See current implementation for Edit/Delete file in !170512 (merged). For consistency, these changes will also need to be percolated to other areas where the commit changes modal is surfaced: New file, Upload file, and New directory.

Steps

1. Go to "Code > Repository"
2. Select a file
3. Click "Edit"

Scenarios

The following conditions should be considered (see !170512 (merged)):

• Commit to the current branch and user can push to the branch
• Commit to the current branch and user can NOT push to the branch
• Commit to new branch and open a MR
• Commit to new branch and NOT open a MR
• Edit LFS file
• Edit while collaboration on merge requests across forks
• Editing a file that has been edited by another user as a user with permission
• Editing a file that has been edited by another user as a user without permission
• Edit file from merge request
• Cancel button pops a confirmation modal
• Mobile screen

Related UI

These are areas of the UI that should all be using the same commit changes modal UI:

UI element	Issues/MRs	Reproduction steps
Edit file	#444302 (closed) (this) !170512 (merged) (in review)	Code > Repository > Select a file > Edit > Edit single file
Delete file	#444302 (closed) (this) !168992 (merged) #503405 (closed) (open)	Code > Repository > Select a file > Delete
New file	#502460 (closed) (open)	Code > Repository > New file
Upload file	#479249 (closed) (open)	Code > Repository > Upload file
New directory	#502483 (closed) (open)	Code > Repository > New directory