Replace QRegExp with QRegularExpression

Summary:
Note: RFC 2369 advises client applications to ignore whitespace inside
the angle brackets, so I added `\s*` to the mailto regexp in some
places.  fromUnicodeString() seems to handle whitespace in the mail
address.

Test Plan: Run replystrategytest and messagefactoryngtest.

Reviewers: #kde_pim, dvratil

Reviewed By: #kde_pim, dvratil

Subscribers: dvratil, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D29241