Add Validation to Forms

[xarrijorge]

At the moment, the forms are not throwing any errors if the wrong are inserted. Add a function to alert users to input a proper email or have a secure password or username.

[junyada100]

i've planned to put those client validation functions in static/js/func.js, and using it by calling from static/js/signin.js + signup.js specifically on each pages; I hope this way can help to make it clear and easy to understand/re-using them.