luci-base: luci.js: add LuCI.session.getToken()

Since we're already have LuCI.session.getID() we should mirror the same
interface for retrieving the session token.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(cherry picked from commit 551d839f90)

modules/luci-base/htdocs/luci-static/resources/luci.js

@@ -1844,6 +1844,16 @@
 			return env.sessionid || '00000000000000000000000000000000';
 		},
 
+		/**
+		 * Retrieve the current session token.
+		 *
+		 * @returns {string|null}
+		 * Returns the current session token or `null` if not logged in.
+		 */
+		getToken: function() {
+			return env.token || null;
+		},
+
 		/**
 		 * Retrieve data from the local session storage.
 		 *