Implements the form for renaming the new filename on the file edit page

## What does this MR do?
Adds the possibility to edit a filename when editing the content of a file on Gitlab

this MR was made to solve #5850 

![Screen_Shot_2016-07-01_at_16.45.53](/uploads/eeed05800e3dc11f1e3840e0e4fa06b5/Screen_Shot_2016-07-01_at_16.45.53.png)

See merge request !5049