Mentioning should check if the user exists

[kolaente]

• Gitea version (or commit ref): 1.4.0-rc1
• Can you reproduce the bug at https://try.gitea.io:
  • Probably

Description

When mentioning someone in an issue or pr, it doesn't seem to matter if the user exists or not. For example, when I mention someone @NonexistentUser, Gitea still creates a link from it, leading to a 404 page. It shold check if the user exists and only create the link if it does.

[bkcsoft]

If someone implements this, cache the result for at least that page-view