Add option for CRAP proteins to custom databases

[pverscha]

Reported by @tivdnbos.

The CRAP database contains protein that are typically found as contaminants when working with protein samples (they originate from e.g. hair, human cells, ...). These proteins are present in UniProt and thus taken into account when working with the unfiltered database, but they are not taken into account when users are building there own custom database.

There should be an option in the Unipept Desktop app to add these CRAP-proteins to a custom database.

[pverscha]

The protein trypsine is not present in CRAP but also needs to be taken into account in this regard: P00761.