Refactor UserInternalRegexHandler to own module

What does this MR do?

This MR refactors a class which was exported from a page module. Previously:

• The tests which imported this module had a side-effect on import
• The class used jquery

With this MR:

• This class has been refactored into a function that lives in a module outside the pages directory.
• It no longer uses jquery.

Screenshots (strongly suggested)

Everything still works! 🎉

20210317_admin_users_new_demo

• When email matches "internal user regex", then we automatically uncheck the "external" checkbox and show a warning message
• This warning message is hidden when the checkbox is changed by the user

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
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  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content