Create dedicated SASL commands in AdminRoomHandler

[emersonveenstra]

Is your feature request related to a problem? Please describe.
Right now !username and !password are overloaded, they try to set both the IRC username/password and SASL username/password. This is responsible for several issues in here (#1421, #1361, #1359, #1350, etc) and I think the easiest solution is to introduce !saslUsername and !saslPassword to the admin room config options, to separate it completely from the other options.

I just made matrix-org/node-irc#80 that allows saslUsername and saslPassword to be sent as options to node-irc, so once that gets merged then someone can implement it in here.

Just to clarify, with this idea:
!username would only store the desired name for the user part of the nick!user@host on IRC
!password would store a string to be sent only with the PASS command on initial connection
!saslUsername and !saslPassword would store strings to be used only when the server supports SASL.

We'd probably need a bit of deprecation time where both would be allowed as well.