www/gitea: Update to 1.8.0

Changelog: https://blog.gitea.io/2019/04/gitea-1.8.0-is-released/

• Add UPDATING

PR: 237734
Submitted by: stb@lassitu.de (maintainer)
MFH: 2019Q2
Security: a1de4ae9-6fda-11e9-9ba0-4c72b94353b5
Sponsored by: Netzkommune GmbH