[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 42f964fd18

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>

templates/status/404.tmpl

@@ -1,7 +1,7 @@
 {{template "base/head" .}}
 {{if .IsRepo}}<div class="repository">{{template "repo/header" .}}</div>{{end}}
 <div class="ui container center">
-	<p style="margin-top: 100px"><img src="{{AppSubUrl}}/img/404.png" alt="404"/></p>
+	<p style="margin-top: 100px"><img src="{{StaticUrlPrefix}}/img/404.png" alt="404"/></p>
 	<div class="ui divider"></div>
 	<br>
 	{{if .ShowFooterVersion}}<p>Application Version: {{AppVer}}</p>{{end}}

templates/status/500.tmpl

@@ -1,6 +1,6 @@
 {{template "base/head" .}}
 <div class="ui container center">
-	<p style="margin-top: 100px"><img src="{{AppSubUrl}}/img/500.png" alt="500"/></p>
+	<p style="margin-top: 100px"><img src="{{StaticUrlPrefix}}/img/500.png" alt="500"/></p>
 	<div class="ui divider"></div>
 	<br>
 	{{if .ErrorMsg}}<p>An error has occurred :</p>