feature: add group main series id

[revam]

and tie it into the auto grouper feature.

[revam]

Will do the TreeController tomorrow.

[revam]

Also blocked by https://github.com/ShokoAnime/Shoko.Commons/pull/8 and https://github.com/ShokoAnime/Shoko.Models/pull/7

[revam]

I didn't write any migration code because I'm unsure how to handle it for existing series when a user have set up a lot of custom groups, and just running the "Recreate all groups" action will add the auto main id to the groups for those using automatically generated groups.