Split the routines to convert database fields to UUIDs into separate methods
Rename `updateDatabaseTables()` to `adjustDatabaseTables()`

The method will otherwise be called to early by the `runDatabaseUpdates()` method, which calls all methods starting with "update".
Enable the safe mode and maintenance mode during the database updates
Use `null` instead of `0` in `tl_files.pid`
Mark the backup database fields in the database updater
Merge branch 'hotfix/database-update' into release/3.2.0