Fortune: Introduce fortune display

Add a new sub-module that displays Fortune in character and party sheets.
In the character sheet, this is an input field and is automatically tracked
thanks to flags.

Add an icon for the party sheet and the according CSS class.

styles/mills_fabula.css

@@ -0,0 +1,3 @@
+.mf-fortune {
+    background-image: url("/modules/the-mills-fabula/assets/fortune.png");
+}