In Wiki pages, short-links created to local Wiki files were always expanded
as regular Wiki Links. In particular, if a link wanted to point to a file
that Gitea doesn't know how to render (e.g, a .zip file), a user following
the link would be silently redirected to the Wiki's home page.

This change makes short-links* in Wiki pages be expanded to raw wiki links,
so these local wiki files may be accessed without manually accessing their URL.

* only short-links ending in a file extension that isn't renderable are affected.

Closes go-gitea#27121.

Signed-off-by: Rafael Girão <rafael.s.girao@tecnico.ulisboa.pt>