Support components that were re-created in gretel-upload

[tijmenbaarda]

The backend currently supports user-uploaded components by adding them to the Django database on the fly, and by doing that it counts the number of sentences and words for the sake of progress measurement, but it does not test if these components have been changed or re-created in the meantime.

This can be solved by checking if the last-changed date of the BaseX database is later than the creation date of the component entry in the Django database. If so, the database entry has to be updated. This also needs to happen if a cached query is available.