Artist discography - Use Musicbrainz releasegroup ids for matching

[DaveTBlake]

Improve artist discography matching to albums in users music library by using Musicbrainz release group id when the artist scraper provides it.

Bump music db to v79 for change to discography table

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Sometimes artist and album title are not enough to uniquely identify an album e.g. there are 5 different albums (with totally different tracks) by Peter Gabriel called "Peter Gabriel". In Musicbrainz these are referenced different release groups, and collectors that have name duplicate albums like these can add them all to their music library providing that they tag the music files with mbid tags.

However the artist discography facility - a scraped list of albums by the artist independant of what albums the user has in their library - was not able to handle these name duplicates when matching them to those that where in the library. This PR fixes that.

[DaveTBlake]

This has been tested in conjunction with scrapers that set release group mbid for discography entries:
xml scraper Universal Artist Scraper v4.3.3 and updated dependency MB common scraper v2.2.4
and a development version of the Python scraper Generic Artists Scraper v 1.0.6

Used a library with albums with and without mbid values, on artists with discography entries that are not unique by name, and with some of them in the library as albums etc.

[Rechi]

Please follow the current code guidelines.