Replace Roadmap link with Planned Features

The "roadmap" issue boards on GitLab aren't being maintained in any sort
of useful way, and are probably just more confusing to anyone than
helpful. This replaces that link with a "Planned Features" one that goes
to an issue search for ones that have both the "Stage::Accepted" and
"Feature Request" labels, sorted by GitLab's "priority" method which
will put High Priority ones at the top.

tildes/tildes/templates/base.jinja2

@@ -138,7 +138,7 @@
     <li class="site-footer-link"><a href="https://blog.tildes.net">Blog</a></li>
     <li class="site-footer-link"><a href="https://docs.tildes.net/contact">Contact</a></li>
     <li class="site-footer-link"><a href="https://gitlab.com/tildes/tildes">Source code</a></li>
-    <li class="site-footer-link"><a href="https://gitlab.com/tildes/tildes/issues">Issues</a> / <a href="https://gitlab.com/tildes/tildes/boards">Roadmap</a></li>
+    <li class="site-footer-link"><a href="https://gitlab.com/tildes/tildes/issues?label_name[]=Feature+Request&label_name[]=Stage%3A%3AAccepted&scope=all&sort=priority&state=opened">Planned features</a> / <a href="https://gitlab.com/tildes/tildes/issues">All issues</a></li>
     <li class="site-footer-link"><a href="https://docs.tildes.net/policies/privacy-policy">Privacy policy</a></li>
     <li class="site-footer-link"><a href="https://docs.tildes.net/policies/terms-of-use">Terms of use</a></li>
   </ul>