Czech translation

First attempt at Czech translation. Note that some words don't have
a good and short translation, e.g. "peers", "pre-shared key" or
"persistent keepalives". In these cases, some approximate/familiar
"Czech-English" amalgamation is used.

src/static/locale/active_languages.json

@@ -28,10 +28,15 @@
 		"lang_id": "it-IT",
 		"lang_name": "Italian",
 		"lang_name_localized": "Italiano"
-  },
-  {
+	},
+	{
 		"lang_id": "ru",
 		"lang_name": "Russian",
 		"lang_name_localized": "Русский"
-	}
+	},
+	{
+		"lang_id": "cs",
+		"lang_name": "Czech",
+		"lang_name_localized": "Česky"
+	},
 ]