views: Integrate cover letter support

views: Integrate cover letter support

There isn't really any need to list cover letters right now, seeing as
they're really only valuable in the context of series. However, if
someone requests a cover letter by ID then this should be displayed.
To this effect, add a new "covers" endpoint that can display the basic
elements of the cover letter. This includes redirects from/to the
"patches" endpoint.

Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
Reviewed-by: Andy Doan <andy.doan@linaro.org>