prevent rename from creating similarly named files on the same provider, user and group with rename

[MontrealSergiy]

One can upload or register a few files to the same provider and the rename both to same name. This should not be possible, cbrain files should have different names step to reproduce

1. register a few files
2. select one file, click rename and enter the name of second followed by enter observed: files with a same name expected: error message