The logic of "URLJoin" is unclear and it is often abused.
Also:
* Correct the `resolveLinkRelative` behavior
* Fix missing "PathEscape" in `ToTag`
* Fix more FIXMEs, and add new FIXMEs for newly found problems
* Refactor "auth page common template data"
* Fix#33972
* Use consistent path resolving for links and medias.
* No need to make the markup renders to resolve the paths, instead, the
paths are all correctly resolved in the "post process" step.
* Fix#33274
* Since 1.23, all paths starting with "/" are relative to current render
context (for example: the current repo branch)
* Introduce `/:root/path-relative-to-root`, then the path will be
rendered as relative to "ROOT_URL"
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)
