Simplified User name regular expression

[snittoor]

Simplified the regular expression evaluation for username.These are the features:-
/^(?!-)(?!)[a-zA-Z0-9-]{3,15}$/;

1. [a-zA-Z0-9-_] :- The username is alphanumeric and can begin with a number and can have a hypen or underscore
2. (?!-)(?!_):- The username cannot begin with a hypen or underscore.
3. {3,15}:- Minimum (3-15) in length.

Closes #1985
Closes #1963

[bajiat]

@snittoor Is this PR work in progress or ready for review?

[snittoor]

Ready for review