REST: Add '/series' endpoint

REST: Add '/series' endpoint

Adopt a hybrid approach, by adding an additional series endpoint to the
existing patch endpoint:

    /series/${series_id}/
    /patches/${patch_id}/

This is based on the approach described here:

    http://softwareengineering.stackexchange.com/a/275007/106804

This is necessary due to the N:N mapping of series and patches: it's
possible for a patch to belong to many series, and a series usually
contains many patches. This means it is not possible to rely on the
patch endpoint alone.

It is also necessary to add a cover letter endpoint, such that the
series body can include this.

Signed-off-by: Stephen Finucane <stephen@that.guru>