Update "Submit" -> "Save" as discussed in #3319 (#3325)

In German UI, "Submit" (Abschicken) is almost never used. Instead, people are used to have "Save" (Speichern) as text on their buttons.
https://github.com/FreshRSS/FreshRSS/issues/3319

app/i18n/de/gen.php

@@ -24,7 +24,7 @@ return array(
 		'remove' => 'Entfernen',
 		'rename' => 'Umbenennen',
 		'see_website' => 'Website ansehen',
-		'submit' => 'Abschicken',
+		'submit' => 'Speichern',
 		'truncate' => 'Alle Artikel löschen',
 		'update' => 'Aktualisieren',
 	),