Introduce jquery formvalidator for com_plugins

#### Executive summary

This PR converts the form validation on com_plugins 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_plugins 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