`http.ServeFile/ServeContent` handles `If-xxx`, `Content-Length`,
`Range` and `Etag` correctly

After this PR, storage files (eg: avatar) could be responded with
correct Content-Length.

Instead of adding it

# Before
On the raw commit page:

![image](https://user-images.githubusercontent.com/20454870/223470744-cdf11898-e023-4198-8c8b-c294e5d78b73.png)

# After

![image](https://user-images.githubusercontent.com/20454870/223470596-af898d66-bd5b-4ddb-b220-ceb1f149bfec.png)

Fixes #23308

---------

Signed-off-by: Yarden Shoham <hrsi88@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: John Olheiser <john.olheiser@gmail.com>

When using the main branch, I found that some changed code didn't have
comments.

This PR adds some comments.

The `no-store` cache control added in #20432 is causing form input to be
cleared unnecessarily on page reload. Instead use
`max-age=0,private,must-revalidate` which avoids this.

This was particularly a problem when typing a long comment for an issue
and then for example changing the label. The page would be reloaded and
lose the unsubmitted comment.

Fixes #22603

Change all license headers to comply with REUSE specification.

Fix #16132

Co-authored-by: flynnnnnnnnnn <flynnnnnnnnnn@github>
Co-authored-by: John Olheiser <john.olheiser@gmail.com>

`no-transform` allegedly disables CloudFlare auto-minify and we did not
set caching headers on html or api requests, which seems good to have
regardless.

Transformation is still allowed for asset requests.

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Andrew Thornton <art27@cantab.net>

Although the use of LastModified dates for caching of git objects should be
discouraged (as it is not native to git - and there are a LOT of ways this
could be incorrect) - LastModified dates can be a helpful somewhat more human
way of caching for simple cases.

This PR adds this header and handles the If-Modified-Since header to the /raw/
routes.

Fix #18354

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: 6543 <6543@obermui.de>

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.

* Add ETag header.

* Comply with RFC 7232.

* Moved logic into httpcache.go

* Changed name.

* Lint

* Implemented If-None-Match list.

* Fixed missing header on *

* Removed weak etag support.

* Removed * support.

* Added unit test.

* Lint

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: zeripath <art27@cantab.net>

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.