Automatically fork a project when not allowed to edit a file.
Fixes#3215.
To do:
- [ ] Add tests
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## "Edit" button on file in a project the user does NOT have write access to
## Clicking will automatically create a fork
## When the fork has been created, the user is returned to the edit page on the original project with a notice
## The user cannot change the target branch and is informed that editing will start an MR
## Hitting "Commit changes" will commit and start an MR from my fork to the origin project
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## "Create file, "Upload file" and "New directory" buttons in a project the user does NOT have write access to
## Clicking any of these options will automatically create a fork
## When the fork has been created, the user is returned to the tree page on the original project with a notice
## Clicking "New directory" again will show the modal. The user cannot change the target branch and is informed that editing will start an MR
## Hitting "Create directory" will commit and start an MR from my fork to the origin project
cc @dzaporozhets @skyruler
See merge request !2145
76b7e24f85