DEV: Trigger 'user_added_to_group' when accepting invite with groups (PR #9561)

When a user accepts an invite that automatically adds them to 1 or more groups, the DiscourseEvent for user_added_to_group is never triggered.

This is because the trigger is only called inside Group#add https://github.com/discourse/discourse/blob/01f2819ddec1f0291cbe67bbfa76d41146a986aa/app/models/group.rb#L645

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