Replace flattr with gratipay

Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/html/about.html

@@ -21,7 +21,7 @@
 
 </div>
 <div class="panel-footer">
-<a class="btn btn-default" href="https://flattr.com/thing/571085/Weblate">{% trans "Flattr Weblate!" %}</a>
+<a class="btn btn-default" href="https://gratipay.com/nijel/">{% trans "Fund Weblate!" %}</a>
 <a class="btn btn-default" href="https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=AXN88S8GX5MQ4">{% trans "Donate to Weblate!" %}</a>
 </div>
 </div>