Don't require credentials in DB client

[Mr0grog]

The web-monitoring-db client in web_monitoring/db.py currently requires credentials, but the HTTP API no longer requires authentication for some operations (edgi-govdata-archiving/web-monitoring-db#1069). Accordingly, the client here shouldn’t require an e-mail and password in order to use.

• Make email and password constructor parameters optional.
• Raise if only one of email and password has a value.
• Don’t set auth credentials if email and password are not present.
• Don’t require any of the env vars to be present in .from_env().
• Consider adding a separate exception for 403 errors (we have one for 401 — not authenticated — but not 403 — forbidden/not authorized). Users without credentials will probably encounter 403 errors more often than 401 now.