models: Use parent model to get comment's 'list_archive_url'

models: Use parent model to get comment's 'list_archive_url'

We were attempting to retrieve the 'list_archive_url' attribute from the
'PatchComment' or 'CoverComment' instances, rather than the parent
'Patch' and 'Cover' object, respectively. Correct this and add plenty of
tests to prevent this regressing.

NOTE(stephenfin): Squashed in 5fe02b6a7444b40f705d1efe33e70a9a9ef36e0a
which added the release note for this fix.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Fixes: 02ffb1315 ("models: Add list archive lookup")
Closes: #391
(cherry picked from commit 93ff4db29262c0560122f61eadf78d9626def238)