Enhance email regex to catch common errors

[rtibbles]

Summary

Description of the change(s) you made

Adds some extra restrictions to the email validation to protect against < and > being wrapped around email addresses.

Adds many tests to ensure we are not being too restrictive of valid email addresses, adds some tests of other emails we can safely exclude.

Examples drawn from here: https://en.wikipedia.org/wiki/Email_address#Examples

Manual verification steps performed

Unit tests

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Reviewer guidance

How can a reviewer test these changes?

Check that valid email addresses can still have channels shared with them.
Check that <this@this.com> cannot be shared with.

References

Fixes #3021

Contributor's Checklist

Testing:

• Code is clean and well-commented
• Contributor has fully tested the PR manually
• Critical and brittle code paths are covered by unit tests

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Reviewer's Checklist

This section is for reviewers to fill out.

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[marcellamaki]

Looks good to me! (And good email resource - bookmarking that for future reference!)

[pcenov]

Tested at hotfixes.studio.learningequality.org - both sending invitations and the new email validation function correctly and as described.