Fix email regex

[benpaxton-hf]

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• Tests exist or have been written that cover this particular change.

Change Details:

• Unescape a (?: in the email regex that shouldn't be escaped
• Add a ) in the correct position to close the above
• Add additional tests to verify correctness of email regex (both new tests fail with the old regex)

GitHub seems to not show the diff very nicely; the previously-escaped (?: is just before the second instance of x01 in the regex; the new ) is just before the @. git show <commit> --word-diff=color --word-diff-regex=. may help view this diff properly.

@go-playground/admins

[coveralls]

Coverage remained the same at 100.0% when pulling 9099388 on benpaxton-hf:fix-email-regex into b199fa0 on go-playground:v9.

[deankarn]

Thanks @benpaxton-hf for the PR, will try to look at it this week