REVIEW: [NEXUS-5420] Prepend base URL to all emails (Take 2)

https://issues.sonatype.org/browse/NEXUS-5420
https://bamboo.zion.sonatype.com/browse/NXO-OSSF31

Beside solving the issue, the pull introduces two new Nexus specific Mail types. When used those will prepend the email with Nexus base url. User still can use the micromailer default/html types that will work as before (no prepend).

The templates are read from VM files.