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)
Remove unmaintainable sanitizer rules. No need to add special "class"
regexp rules anymore, use RenderInternal.SafeAttr instead, more details
(and examples) are in the tests
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
