Case insensitive username

[inderpreetsingh]

I was looking into open source slack alternatives for a little startup and came across rocket.chat. I deployed it locally and when I tried to signup with username IshwerDAS (that's how we would like to spell it as DAS is actually an acronym). Anyway, it gave me an error. It took me a while to figure out what was wrong as all that error said was use only letters, numbers, dots and dashes and that was all I was using. A little look into code made me realize that regex used was only for lower case letters. So now either the error given should have been better or it should accept mixed case usernames. I preferred later and edited code to accept mixed case usernames. But I also didn't want 2 people signing up with same username in different case, so changed the function usernameIsAvaliable to be case insensitive when searching.

See PR #341

Other solutions could be
*Change the error message for better
*Convert whatever user types in username box to lowercase before submitting.