ui: improve error pages (#7274)

* add testing
* make each page accessible via `/devtest/error`
* allow translating the `Page not found` part of the title
* code: improve consistency, remove unused
* devtest: put index page in a container to fix alignment
* 500: make navbar more like the real one, remove fake menu button
* deadcode: remove unused `func NotFound`: it was added in bdd32f152d and the only usage was removed in 1bfb0a24d8

Preview:
https://codeberg.org/attachments/1b75afb3-e898-410f-be02-f036a5400143

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7274
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Reviewed-by: Beowulf <beowulf@beocode.eu>

routers/web/web.go

@@ -1,4 +1,5 @@
 // Copyright 2017 The Gitea Authors. All rights reserved.
+// Copyright 2023 The Forgejo Authors. All rights reserved.
 // SPDX-License-Identifier: MIT
 
 package web
@@ -1661,6 +1662,7 @@ func registerRoutes(m *web.Route) {
 		m.Any("/devtest", devtest.List)
 		m.Any("/devtest/fetch-action-test", devtest.FetchActionTest)
 		m.Any("/devtest/{sub}", devtest.Tmpl)
+		m.Get("/devtest/error/{errcode}", devtest.ErrorPage)
 	}
 
 	m.NotFound(func(w http.ResponseWriter, req *http.Request) {