Use raw Wiki links for non-renderable Wiki files (#30273)

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 #27121.

Signed-off-by: Rafael Girão <rafael.s.girao@tecnico.ulisboa.pt>
Co-authored-by: silverwind <me@silverwind.io>

tests/gitea-repositories-meta/user2/repo1.wiki.git/objects/a5/bbc0fd39a696feabed2d4cccaf05abbcaf3b02

@@ -0,0 +1 @@
+x
<01><>Kn<4B> D<><44>죱<>v<EFBFBD>EQ<45><51>~n<>@3F<33><0B><>r<EFBFBD>\,d<><64>^<5E>T<EFBFBD><54>S<EFBFBD>ϏGj|<7C><>K+D<><44><EFBFBD>
<01>

tests/gitea-repositories-meta/user2/repo1.wiki.git/refs/heads/master

@@ -1 +1 @@
-0dca5bd9b5d7ef937710e056f575e86c0184ba85
+a5bbc0fd39a696feabed2d4cccaf05abbcaf3b02