Fix compatibility issues with older versions of Review Board.

Versions prior to 1.7.8 didn't have a 'url' field, causing
the 'absolute_url' property to fail at generated a suitable URL. We now
have a custom 'url' property on ReviewRequestResource that provides a
suitable default, just like 'absolute_url' does.

Likewise, versions of Review Board prior to 1.7.19 didn't have a
'mirror_path' field on the Repository resource, which broke a lookup. We
now conditionally get the 'mirror_path' instead of assuming it's there.
This is done instead of providing a custom property, in order to prevent
the resource from being misleading (since we can't generate any sane
default there).

Fixes bug #3348
Reviewed at https://reviews.reviewboard.org/r/5963/