Added option for file checks only

This is built on top of !3 (merged) and !4 (merged) and should be reviewed after these got in (or if you want to check it out before, I suggest looking on the last commit only).

This MR introduces a command-line argument to do file checks only. This is useful for checks without having a connection to the actual database. Maybe something to add in the CI pipelines of the database-* repositories.

\approve @ds @prehn