Do not allow special chars in usernames

[ihsaan-ullah]

@Didayolo @ObadaS

A brief description of the purpose of the changes contained in this PR.

Special characters in username are problematic when django creates URLs. This PR restricts users to not use special characters in their usernames during signup

Issues this PR resolves

• Issue to log in #1780

A checklist for hand testing

• Check that you cannot create an account if you have special character, capital letter in your username

Checklist

• Code review by me
• Hand tested by me
• I'm proud of my work
• Code review by reviewer
• Hand tested by reviewer
• CircleCi tests are passing
• Ready to merge

[ObadaS]

Wouldn't it be better, if possible, to allow special character in the name but change the way Django creates URL to "sanitize" the input first, removing the special characters ?

With this fix, people that have special characters in their name won't be to create an account without having to remove them.

[ihsaan-ullah]

I think special characters should not be allowed in usernames. This was the case in the code but it was not working. In this PR I have changed the code to fix that code.