Introduce the treasurer role

This role is given to people who are responsible for keeping the money and
accounting. Functions related to setting up accounting and actual booking
are limited to these role.

src/main/java/com/penguineering/gartenplus/auth/role/SystemRole.java

@@ -6,7 +6,8 @@
 public enum SystemRole {
     ADMINISTRATOR("administrator", "Administrator"),
     MEMBER("member", "Gartenfreund"),
-    FRIEND("friend", "Gartenfreund-Freund");
+    FRIEND("friend", "Gartenfreund-Freund"),
+    TREASURER("treasurer", "Geldhüter");
 
     private final String handle;
     private final String displayName;