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).

New unit tests pass.