Added logout button

Logout button was added to Web UI.

Click on logout button executes session_logout command. If command succeeds or xhr stutus is 401 (unauthorized - already logged out) page is redirected to logout.html.

logout.html is a simple page with "You have been logged out" text and a link to return back to main page.

https://fedorahosted.org/freeipa/ticket/2363

ipalib/plugins/internal.py

@@ -245,7 +245,9 @@ class i18n_messages(Command):
         },
         "false": _("False"),
         "login": {
-            "header": _("Logged In As")
+            "header": _("Logged In As"),
+            "logout": _("Logout"),
+            "logout_error": _("Logout error"),
         },
         "objects": {
             "aci": {