Add post-persistence global mutation to store
Problem: every time we change something in store persistent state will load old data. This was especially the case in Themes v2 and now it's in MastoAPI registration MR.
Current workaround: check what state is and work with that even if it's not exactly documented or mentioned in code
Proposed solution: add a "persistent state loaded" type of migration that would tell all vuex modules to check for old stuff and migrate it if needed.