[2.8.1 Backport] ISGR credentials with characters requiring URL encoding cause blipsync to fail

Credentials for ISG replications are incorrectly escaped with URL encoding before being base64 encoded when building the Authorization header for the _blipsync endpoint.

This results in websockets failing to connect if usernames or passwords contain a lot of special characters due to the reason: "bad status", even though the prior HTTP request succeeded (the initial HTTP request is using basic auth, and so escaping there is a valid thing to do).