Only show unique checks

Only show unique checks

Commit e5c641fc4 optimized fetching of checks when displaying a patch by
prefetching these checks ahead of time. Unfortunately we missed that
this should exclude older versions of checks for a given context. Make
the code used to generate the unique checks generic and allow us to use
that as a filter to the checks provided to the template, restoring the
correct behavior.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Closes: #398
Fixes: e5c641fc4 ("Optimise fetching checks when displaying a patch")
(cherry picked from commit a43d6acda0939c14e8135549968815876680d497)