[assets] configurable URL for static resources (#7911)

* static url

* add cors support for static resources

* [assets] work on the migration to configurable url for assets

Signed-off-by: Jakob Ackermann <das7pad@outlook.com>

* [misc] fix whitespace

Signed-off-by: Jakob Ackermann <das7pad@outlook.com>

* [assets] fix the loading of the manifest.json

It is generated dynamically, and as such can not be served by the cdn.

Signed-off-by: Jakob Ackermann <das7pad@outlook.com>

* Revert "add cors support for static resources"

This reverts commit 42f964fd181dbb8b139808b9be623470d4f0e40f

Signed-off-by: Jakob Ackermann <das7pad@outlook.com>

* [docs] add the STATIC_URL_PREFIX option

Signed-off-by: Jakob Ackermann <das7pad@outlook.com>

* [docs] reverse-proxy: nginx: add two setups for STATIC_URL_PREFIX

Signed-off-by: Jakob Ackermann <das7pad@outlook.com>

* [assets] migrate the url of a new asset to the static url prefix

REF: f2a3abc683
Signed-off-by: Jakob Ackermann <das7pad@outlook.com>

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -138,6 +138,13 @@ Values containing `#` or `;` must be quoted using `` ` `` or `"""`.
 - `ROOT_URL`: **%(PROTOCOL)s://%(DOMAIN)s:%(HTTP\_PORT)s/**:
    Overwrite the automatically generated public URL.
    This is useful if the internal and the external URL don't match (e.g. in Docker).
+- `STATIC_URL_PREFIX`: **\<empty\>**:
+   Overwrite this option to request static resources from a different URL.
+   This includes CSS files, images, JS files and web fonts.
+   Avatar images are dynamic resources and still served by gitea.
+   The option can be just a different path, as in `/static`, or another domain, as in `https://cdn.example.com`.
+   Requests are then made as `%(ROOT_URL)s/static/css/index.css` and `https://cdn.example.com/css/index.css` respective.
+   The static files are located in the `public/` directory of the gitea source repository.
 - `HTTP_ADDR`: **0.0.0.0**: HTTP listen address.
    - If `PROTOCOL` is set to `fcgi`, Gitea will listen for FastCGI requests on TCP socket
      defined by `HTTP_ADDR` and `HTTP_PORT` configuration settings.

docs/content/doc/usage/reverse-proxies.en-us.md

@@ -44,6 +44,74 @@ server {
 
 Then set `[server] ROOT_URL = http://git.example.com/git/` in your configuration.
 
+##  Using Nginx as a reverse proxy and serve static resources directly
+We can tune the performance in splitting requests into categories static and dynamic. 
+
+CSS files, JavaScript files, images and web fonts are static content.
+The front page, a repository view or issue list is dynamic content.
+
+Nginx can serve static resources directly and proxy only the dynamic requests to gitea.
+Nginx is optimized for serving static content, while the proxying of large responses might be the opposite of that
+ (see https://serverfault.com/q/587386).
+
+Download a snap shot of the gitea source repository to `/path/to/gitea/`.
+
+We are only interested in the `public/` directory and you can delete the rest.
+
+Depending on the scale of your user base, you might want to split the traffic to two distinct servers,
+ or use a cdn for the static files.
+
+### using a single node and a single domain
+
+Set `[server] STATIC_URL_PREFIX = /_/static` in your configuration.
+
+```
+server {
+    listen 80;
+    server_name git.example.com;
+
+    location /_/static {
+        alias /path/to/gitea/public;
+    }
+
+    location / {
+        proxy_pass http://localhost:3000;
+    }
+}
+```
+
+### using two nodes and two domains
+
+Set `[server] STATIC_URL_PREFIX = http://cdn.example.com/gitea` in your configuration.
+
+```
+# application server running gitea
+server {
+    listen 80;
+    server_name git.example.com;
+
+    location / {
+        proxy_pass http://localhost:3000;
+    }
+}
+```
+
+```
+# static content delivery server
+server {
+    listen 80;
+    server_name cdn.example.com;
+
+    location /gitea {
+        alias /path/to/gitea/public;
+    }
+
+    location / {
+        return 404;
+    }
+}
+```
+
 ## Using Apache HTTPD as a reverse proxy
 
 If you want Apache HTTPD to serve your Gitea instance, you can add the following to your Apache HTTPD configuration (usually located at `/etc/apache2/httpd.conf` in Ubuntu):