Strip locked MUC domain from room address input

[mwild1]

Is your feature request related to a problem? Please describe.

1. Deploy a configuration that has a locked MUC domain, e.g. muc.example.org
2. Click Add a new groupchat
3. Enter a room address like room@muc.example.org
4. The @ gets escaped and Converse.js attempts to join room\40muc.example.org@muc.example.org

Describe the solution you'd like

If the input ends with @<locked domain> then it should be stripped before further processing.

Rationale

If the entered address ends with @<locked domain> it is almost certainly not the user's intention to have the full input as the room name, but means the user entered the full address of a room.

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[jcbrand]

@bernard-ng: Something you'd be willing to look into?

[bernard-ng]

@bernard-ng: Something you'd be willing to look into?

Yes gladly