Updates to FileCabinet tag union SQL statements
Removed needless FileCabinet modification on file rename
Corrected implementation of file rename/tag removal

- changed semantics of add_tag_to_file: The File must be inserted before
  adding the tags, and the addition of tags affects the rest of the database
  immediately. Tags added directly to the file are still respected however.
- removed the "homeless" state for files.
Merge branch 'master' of github.com:mwatts15/TagFS