gallery.py: Handle query and path separately for ._url()

gallery.py: Handle query and path separately for ._url()

This avoids computing a relpath for:

  /full/path/to/gallery?pp=2

from a base-path of:

  /full/path/to/gallery

and getting:

  ../gallery?pp=2

instead of the expected:

  ?pp=2

We could avoid this by using gallery/ in the first place, but keeping
query data as a dict until the last possible moment is also nice.