Split 'show boosts/replies in public timelines' glitch admin setting into local and federated (neatchee/mastodon#8)

[neatchee]

Showing all federated boosts is noisy while local boosts may be acceptable (especially once neatchee#4 is addressed)

[ClearlyClaire]

I think this is missing changes in config/settings.yml to change the defaults. In addition, it's pretty minor, but I'd like to have the current setting be automatically migrated to the newer ones… but on this, while this can be done and has been done is the past (see db/migrate/20190314181829_migrate_open_registrations_setting.rb) I'm not super comfortable manipulating Settings like that either. Though I guess this can be revisited in the future if we drop or update the settings gem.

[neatchee]

Alrighty, that should cover all the feedback :)
I don't especially like changing settings that way either but we don't really have much choice in this scenario

[github-actions]

This pull request has merge conflicts that must be resolved before it can be merged.