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
![edit_file](/uploads/7602157420768aef483a6586bba2d164/edit_file.png)
## Clicking will automatically create a fork
![during_fork](/uploads/32f4f5dff9f24ea99522000b0bf881c5/during_fork.png)
## When the fork has been created, the user is returned to the edit page on the original project with a notice
![edit_notice](/uploads/94ed1319404370ff1e9c0d672fb41e03/edit_notice.png)
## The user cannot change the target branch and is informed that editing will start an MR
![edit_footer](/uploads/4da68d4795c7177e575b7c434d16eeae/edit_footer.png)
## Hitting "Commit changes" will commit and start an MR from my fork to the origin project
![Screen_Shot_2015-12-17_at_23.38.08](/uploads/d777a4db6f38a5a1be84031694465bc1/Screen_Shot_2015-12-17_at_23.38.08.png)
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## "Create file, "Upload file" and "New directory" buttons in a project the user does NOT have write access to
![new_directory](/uploads/72f556248f30d6652523bbb4be01b3e0/new_directory.png)
## Clicking any of these options will automatically create a fork
![during_fork](/uploads/32f4f5dff9f24ea99522000b0bf881c5/during_fork.png)
## When the fork has been created, the user is returned to the tree page on the original project with a notice
![new_directory_notice](/uploads/a1a3e11308ae0e8f0913fae6813a37ed/new_directory_notice.png)
## Clicking "New directory" again will show the modal. The user cannot change the target branch and is informed that editing will start an MR
![new_dir](/uploads/99ca8cbfb2f70603e352b3fdf67b6281/new_dir.png)
## Hitting "Create directory" will commit and start an MR from my fork to the origin project
![Screen_Shot_2015-12-17_at_23.39.19](/uploads/3713d0235abf831361b803a6198c5bc1/Screen_Shot_2015-12-17_at_23.39.19.png)
cc @dzaporozhets @skyruler
See merge request !2145