Embed static stylesheets and dependencies

Embedding css and webfont dependencies allows the application to present
itself correctly in an environment that does not allow downloading the
files from a cdn.

Inspiration taken from #1492 but reworked to make use of embed.FS
simplifying the approach.

oauthproxy.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"embed"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -50,6 +51,7 @@ const (
 	oauthCallbackPath = "/callback"
 	authOnlyPath      = "/auth"
 	userInfoPath      = "/userinfo"
+	staticPathPrefix  = "/static/"
 )
 
 var (
@@ -58,6 +60,9 @@ var (
 
 	// ErrAccessDenied means the user should receive a 401 Unauthorized response
 	ErrAccessDenied = errors.New("access denied")
+
+	//go:embed static/*
+	staticFiles embed.FS
 )
 
 // allowedRoute manages method + path based allowlists
@@ -320,6 +325,9 @@ func (p *OAuthProxy) buildProxySubrouter(s *mux.Router) {
 	s.Path(oauthStartPath).HandlerFunc(p.OAuthStart)
 	s.Path(oauthCallbackPath).HandlerFunc(p.OAuthCallback)
 
+	// Static file paths
+	s.PathPrefix(staticPathPrefix).Handler(http.StripPrefix(p.ProxyPrefix, http.FileServer(http.FS(staticFiles))))
+
 	// The userinfo endpoint needs to load sessions before handling the request
 	s.Path(userInfoPath).Handler(p.sessionChain.ThenFunc(p.UserInfo))
 }

pkg/app/pagewriter/error.html

@@ -5,8 +5,8 @@
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=no">
   <title>{{.StatusCode}} {{.Title}}</title>
-<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bulma@0.9.1/css/bulma.min.css">
+<link rel="stylesheet" href="{{.ProxyPrefix}}/static/css/bulma.min.css">
-<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/5.15.2/css/all.min.css">
+<link rel="stylesheet" href="{{.ProxyPrefix}}/static/css/all.min.css">
 
 <script type="text/javascript">
   document.addEventListener('DOMContentLoaded', function() {

pkg/app/pagewriter/sign_in.html

@@ -5,7 +5,7 @@
     <meta charset="utf-8">
     <meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=no">
     <title>Sign In</title>
-    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bulma@0.9.1/css/bulma.min.css">
+    <link rel="stylesheet" href="{{.ProxyPrefix}}/static/css/bulma.min.css">
 
     <style>
       body {