Why this refactor
The goal is to move most files from `models` package to `models.xxx` package. Many models depend on avatar model, so just move this first.
And the existing logic is not clear, there are too many function like `AvatarLink`, `RelAvatarLink`, `SizedRelAvatarLink`, `SizedAvatarLink`, `MakeFinalAvatarURL`, `HashedAvatarLink`, etc. This refactor make everything clear:
* user.AvatarLink()
* user.AvatarLinkWithSize(size)
* avatars.GenerateEmailAvatarFastLink(email, size)
* avatars.GenerateEmailAvatarFinalLink(email, size)
And many duplicated code are deleted in route handler, the handler and the model share the same avatar logic now.
This enabled HTTP time-based cache for storage assets, primarily
avatars. I have not observed If-Modified-Since from browsers during
tests but I guess it's good to support regardless.
It introduces a new generic httpcache module that can handle both
time-based and etag-based caching.
Additionally, manifest.json and robots.txt are now also cachable.
