user can now forget hosts

- hosts are never unpaired, so re-adding the hostname will not require pairing
GFEHostIP field marked as `dirty` when mDNS found
- thanks to https://github.com/google/material-design-lite/issues/903
- untested.

index.html

@@ -54,6 +54,7 @@
                 </div>
                 <div class="button-holder">
                     <button class="mdl-button mdl-js-button mdl-button--raised mdl-button--colored" id="hostChosen">Connect</button>
+                    <button class="mdl-button mdl-js-button mdl-button--raised mdl-button--accent" id="forgetHost">Forget</button>
                 </div>
             </div>