[v1.5.1] Patch release: fix sending email

Mbin patch release v1.5.1 is out, which will fix issues with sending emails (from both the registration form as well as the contact form). Which is very relevant for server admins to upgrade asap.

No other major changes are done in this release.

See also: https://github.com/MbinOrg/mbin/releases/tag/v1.5.1