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>

modules/testlogger/testlogger.go

@@ -363,6 +363,9 @@ var ignoredErrorMessage = []string{
 
 	// TestDatabaseCollation
 	`[E] [Error SQL Query] INSERT INTO test_collation_tbl (txt) VALUES ('main') []`,
+
+	// TestDevtestErrorpages
+	`ErrorPage() [E] Example error: Example error`,
 }
 
 func (w *testLoggerWriterCloser) recordError(msg string) {