Hide nickname changes when previous nickname was like "Anonymous 12345".
Helps to improve performances when massive anonymous users are joining (#138), and prevent displaying unnecessary messages (#111).
* Remember the chosen nickname in sessionStorage, to avoid entering it again too often.
* Fix: if an anonymous chat user enter spaces in the nickname choice, it will allows them to keep the random nickname.
* if both local and remote instance have external XMPP connections enabled, the user joins the remote room with his local account
* some code refactoring (builtin.ts)
Note: documentation and settings descriptions are to do.
Related to #112
