Merge branch 'master' into 'master'

[DNS] Allow free standing @ as hostname

Currently, the regex checking the hostname for adding new DNS records doesn't allow `@` as the hostname. This PR adds support for that for `A`, `AAAA`, `MX` and `TXT` records. Not sure if any other DNS record types allow `@` as hostname, but happy to amend this PR if that's the case.

See merge request !521