Update to 1.7.3

Changelog: https://blog.gitea.io/2019/03/gitea-1.7.4-is-released/

PR: 236563
Submitted by: stb@lassitu.de (maintainer)
MFH: 2019Q1
Security: 8ba7358-4b02-11e9-9ba0-4c72b94353b5
Sponsored by: Netzkommune GmbH