Add sanity checks for configuration on startup

[max-moser]

Context

It can easily happen that some configuration values in InvenioRDM are set to something invalid, leading to
One of the easiest ways of running into such issues is by forgetting to properly copy/symlink invenio.cfg.
Currently, the application will still happily start up, but of course start to fail for certain operations later on with at times quite cryptic error messages.
While over time, operators will learn the symptoms (i.e. the exact error messages resulting from this), it can be quite the head-scratcher for newbies.

Proposed solution

I think it would be beneficial to add some sanity checks for configuration during application startup (e.g. initialization of the invenio-app-rdm extension).
When some required configuration values are missing (or obviously misconfigured), we could then refuse to start up (the nuclear option), or at least print warnings about the configuration issues.