Improve checks for db admin/member
Improve checks for db admin/member

- Use lists:member/2 to check if user name is in list
- Throw forbidden error if user is authenticated on db membership check
- Normalize terminology readers vs members
- Make checks more Erlang-ish

COUCHDB-2534

fkaempfer/couchdb,apache/couchdb-couch,apache/couchdb-couch,cloudant/couchdb-couch,fkaempfer/couchdb,cloudant/couchdb-couch,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,apache/couchdb-couch,cloudant/couchdb-couch,fkaempfer/couchdb