Refactor markup render system

[wxiaoguang]

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)

[lunny]

Move the render under models seems wired. Since only repo_model.Repository has been dependent. Maybe we can create a repository interface and move these render codes to modules/renderer package? We can also extend gitrepo.Repository interface to have more methods.

[wxiaoguang]

Move the render under models seems wired. Since only repo_model.Repository has been dependent. Maybe we can create a repository interface and move these render codes to modules/renderer package? We can also extend gitrepo.Repository interface to have more methods.

There are enough comments, that's the only choice due to the legacy problematic code structure. Otherwise there will be much more unnecessary changes in this PR.