Find manifests by tag name

This MR adds a method to the repository store to find a manifest by tag name, as defined in the API queries spec. This is then used in the manifests handler, instead of two separate queries, one to get the tag and then another to get the manifest.

Unlike with the filesystem metadata, a tag cannot exist without a manifest at the DB level, so in this case, there is never a "manifest not found" error, only a "tag not found" error. If a tag exists, the manifest that it points to is guaranteed to exist as well (because of the foreign key).

Related to #206 (closed)