Add support for navigating to various feeds from a catalog entry

[oxdef]

OPDS spec "5.2.3. Grouping Relations":

An OPDS Catalog Entry may include one or more atom:link elements to Navigation Feeds and Acquisition Feeds which contain it, either directly or indirectly. This specification uses the IANA standard link relation collection for this purpose.
An OPDS client may choose to group together all entries in an OPDS Feed whose collection link points to a given target.

For example, it could be a feed for specific author.