[Task]: Finish migration cleanup #1442

Now that #1432 is merged, a couple of things remain to be done on the migrations:

1. remove Folder stuff
2. in open_sqlite_library replace make_tables with code that creates the tables in the appropriate migrations
3. split migrations out of opening function (preparation for 4. and the subpoint)
   • then split migrations out of library class
4. make migrations sqlalchemy independent