There is no particular reason to set gitea variables globally.
Nonetheless, some misguided packagers set GITEA_CUSTOM in /etc/profile
breaking tests.
Fixes: go-gitea/gitea#36042
This PR removes (almost) all path tricks, and introduces "renderhelper"
package.
Now we can clearly see the rendering behaviors for comment/file/wiki,
more details are in "renderhelper" tests.
Fix#31411 , fix#18592, fix#25632 and maybe more problems. (ps: fix
#32608 by the way)