Introduce jquery formvalidator for com_users

#### Executive summary

This PR converts the form validation on com_users to use plain jquery (no mootools call on every form ).
Also NO MORE INLINE SCRIPTS!

#### Testing

1. Apply first PR #4888 (!important)
2. Apply this PR
3. In the admin area go to com_users and try to submit any form.

If no javascript errors are logged in your browser and the functionality remains the same your test is a pass in any other case please report the errors here

@phproberto I also dislike `$script .=` or `$script[] =` but in this occasion there is a foreach loop that populates some part of the script. I guess I can simplified to three parts:
1. code before the foreach
2. code generated in the loop
3. the rest

Or use `JFactory::getDocument()->addScriptDeclaration(“…”);` 3 times (i am not sure if this is gonna work flawlessly)