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)
To make the markup package easier to maintain:
1. Split some go files into small files
2. Use a shared util.NopCloser, remove duplicate code
3. Remove unused functions
