Wrong topic URL when Gitea installed in the subdomain

[Monter]

• Gitea version (or commit ref): 513db27 and older
• Database:
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • No

Description

When Gitea is installed in the subdomain, "Topics" visible on the home page have the wrong URL to route to the main domain. This error can not be repeated in the trial version of TryGitea because it is not in the subdomain.

[Monter]

-> #5021