Add basic Recording serialization.
Add recording/track serialization to releases, split out duration printing into a role.
ISO duration formatting does not require the inclusion of '0H' when times are less than one hour; remove it.
Add linking to releases from release group pages in JSON-LD.
Clear the cache for the list of all link attribute types when instruments are added/updated/deleted, to ensure it's up to date.
Add a descriptive user-agent string to requests made with the context's LWP::UserAgent instance.
Merge remote-tracking branch 'bitbucket/instrument-clearcache' into beta

* bitbucket/instrument-clearcache:
  Clear the cache for the list of all link attribute types when instruments are added/updated/deleted, to ensure it's up to date.
Merge remote-tracking branch 'bitbucket/musicbrainz-server-ua' into beta

* bitbucket/musicbrainz-server-ua:
  Add a descriptive user-agent string to requests made with the context's LWP::UserAgent instance.
Merge branch 'json-ld-linked-1' into beta

* json-ld-linked-1:
  Add linking to releases from release group pages in JSON-LD.
  ISO duration formatting does not require the inclusion of '0H' when times are less than one hour; remove it.
  Add recording/track serialization to releases, split out duration printing into a role.
  Add basic Recording serialization.