Better way for excluding IdPs reported in the metadata

Right now, we're using a string with comma-separated IdP entity IDs through the environment variable DISCO_HIDDEN_IDP_ENTITY to exclude identity providers. This is fine for a handful of values, but quickly becomes difficult to maintain.

We should look for alternative ways of storing this information, like a JSON file. Information that might be relevant for maintenance:

• Entity ID of the excluded IdP
• Reason for excluding the IdP (likely because it doesn't recognize our SP)
• Date on which we added this entry

Potentially, we could also utilize regular expressions rather than simple lists of entity IDs.

mentioned in merge request !5 (merged)

closed with merge request !5 (merged)

Implemented a regex-based exclude list, read from a file. It supports comments, so any sort of information can be stored.