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.