Add check endpoint which can be used with nginx' auth_request function (#266)

* Add check endpoint which can be used with nginx' auth_request function

* feat(cmd): allow configuring redirect domains

* test: add test environment for the nginx_auth PR

This is a full local setup of the nginx_auth PR including HTTPS so that
it's easier to validate in isolation.

This requires an install of k3s (https://k3s.io) with traefik set to
listen on localhost. This will be amended in the future but for now this
works enough to ship it.

Signed-off-by: Xe Iaso <me@xeiaso.net>

* fix(cmd|lib): allow empty redirect domains variable

Signed-off-by: Xe Iaso <me@xeiaso.net>

* fix(test): add space to target variable in anubis container

Signed-off-by: Xe Iaso <me@xeiaso.net>

* docs(admin): rewrite subrequest auth docs, make generic

* docs(install): document REDIRECT_DOMAINS flag

Signed-off-by: Xe Iaso <me@xeiaso.net>

* feat(lib): clamp redirects to the same HTTP host

Only if REDIRECT_DOMAINS is not set.

Signed-off-by: Xe Iaso <me@xeiaso.net>

---------

Signed-off-by: Xe Iaso <me@xeiaso.net>
Co-authored-by: Xe Iaso <me@xeiaso.net>

web/index.templ

@@ -126,6 +126,18 @@ templ errorPage(message string, mail string) {
 	</div>
 }
 
+templ StaticHappy() {
+<div class="centered-div">
+    <img
+            style="display:none;"
+            style="width:100%;max-width:256px;"
+            src={ "/.within.website/x/cmd/anubis/static/img/happy.webp?cacheBuster=" +
+    anubis.Version }
+    />
+    <p>This is just a check endpoint for your reverse proxy to use.</p>
+</div>
+}
+
 templ bench() {
 	<div style="height:20rem;display:flex">
 		<table style="margin-top:1rem;display:grid;grid-template:auto 1fr/auto auto;gap:0 0.5rem">