Update to 1.7.6

Changelog: https://blog.gitea.io/2019/04/gitea-1.7.6-is-released/

PR: 237303
Submitted by: stb@lassitu.de (maintainer)
MFH: 2019Q2
Security: b747783f-5fb6-11e9-b2ac-08002705f877
Sponsored by: Netzkommune Gmbh