SOFTWARE-5890: Automerge check for unknown institution ID

[mwestphall]

• Add a new check in automerge_check.py that compares the InstitutionID in edited projects files to the list provided by topology-institutions.osg-htc.org/api/institution_ids . Raise the existing ORGS_ADDED error if this occurs.
• Update the error message for ORGS_ADDED to make note of valid institution ID requirement.

[matyasselmeci]

I think this is fine; you should make sure requests is installed into the webhook container but after that, feel free to merge.