Backport #28716 by wxiaoguang
Gitea prefers to use relative URLs in code (to make multiple domain work
for some users)
So it needs to use `toAbsoluteUrl` to generate a full URL when click
"Reference in New Issues"
And add some comments in the test code
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
(cherry picked from commit def178ce32)
Conflicts:
tests/integration/issue_test.go
https://codeberg.org/forgejo/forgejo/issues/2158
01285dc47c