Move public asset files to the proper directory (#25907)

Move `public/*` to `public/assets/*`

Some old PRs (like #15219) introduced inconsistent directory system.

For example: why the local directory "public" is accessed by
`http://site/assets`? How to serve the ".well-known" files properly in
the public directory?

For convention rules, the "public" directory is widely used for the
website's root directory. It shouldn't be an exception for Gitea.

So, this PR makes the things consistent:

* `http://site/assets/foo` means `{CustomPath}/public/assets/foo`.
* `{CustomPath}/public/.well-known` and `{CustomPath}/public/robots.txt`
can be used in the future.

This PR is also a prerequisite for a clear solution for:
* #21942
* #25892 
* discourse.gitea.io: [.well-known path serving custom files behind
proxy?](https://discourse.gitea.io/t/well-known-path-serving-custom-files-behind-proxy/5445/1)

This PR is breaking for users who have custom "public" files (CSS/JS).
After getting approvals, I will update the documents.

----

## ⚠️ BREAKING ⚠️

If you have files in your "custom/public/" folder, please move them to
"custom/public/assets/".

---------

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Giteabot <teabot@gitea.io>

snap/snapcraft.yaml

@@ -6,7 +6,7 @@ description: |
   an independent binary distribution across ALL platforms that Go supports,
   including Linux, Mac OS X, Windows and ARM.
 
-icon: public/img/logo.png
+icon: public/assets/img/logo.png
 confinement: strict
 base: core18
 adopt-info: gitea
@@ -49,7 +49,7 @@ parts:
       - LDFLAGS: ""
     override-pull: |
       snapcraftctl pull
-      
+
       last_committed_tag="$(git for-each-ref --sort=taggerdate --format '%(tag)' refs/tags | tail -n 1)"
       last_released_tag="$(snap info gitea | awk '$1 == "latest/candidate:" { print $2 }')"
       # If the latest tag from the upstream project has not been released to
@@ -57,7 +57,7 @@ parts:
       if [ "${last_committed_tag}" != "${last_released_tag}" ]; then
         git fetch
         git checkout "${last_committed_tag}"
-      fi          
+      fi
 
       version="$(git describe --always | sed -e 's/-/+git/;y/-/./')"
       [ -n "$(echo $version | grep "+git")" ] && grade=devel || grade=stable