Add codemirror keybindings and addons (#311)

* added codemirror addons
- fullScreen
- autorefresh
added a default:
- extraKeys

Co-authored-by: Tilman Vatteroth <tilman.vatteroth@tu-dortmund.de>
Co-authored-by: mrdrogdrog <mr.drogdrog@gmail.com>
Co-authored-by: Erik Michelson <github@erik.michelson.eu>

public/locales/en.json

@@ -204,7 +204,10 @@
         "editorToolbar": {
             "bold": "Bold",
             "italic": "Italic",
+            "underline": "Underline",
             "strikethrough": "Strikethrough",
+            "subscript": "Subscript",
+            "superscript": "Superscript",
             "header": "Heading",
             "code": "Code",
             "blockquote": "Blockquote",