New QA scenario to add/edit/delete a file via the Web UI

Steps:

1. Create a new file via the Web UI in master from a project's home page:
2. Assert that the file has been created and its content is correct
3. Modify the file via the Web UI in master with a new content
4. Assert that the file's content is correct
5. Delete the file via the Web UI
6. Assert that the file has been deleted