Remove jQuery `validate` from codebase

[rebecca-shoptaw]

Closes #9605.

Refactor. The time has come at last!! Following in the footsteps of the previously merged subtasks (see issue page) which replaced each use of the jQuery validate plugin with built-in HTML validation and/or custom JS checks, this PR removes the last traces of the plugin from the codebase.

Technical

Three basic courses of action:

1. Remove the /validate module import in index.js (which activates on any form with a validate class, of which there are now none!)
2. Delete validate.js, which was previously used to set the custom rules for the plugin
3. Uninstall jquery-validation via npm

Testing

1. Search the codebase for validate to confirm that no forms have the class any longer
2. Search the codebase for jquery-validation to confirm that no other files import it
3. Search the codebase for validate again to confirm that no other files import the now-gone validate.js file

Screenshot

Stakeholders

@cdrini