Add offline download to TiddlyWeb control panel

Add button to download an offline-compatible snapshot to the TiddlyWeb
control panel

plugins/tiddlywiki/tiddlyweb/ServerControlPanel.tid

@@ -9,8 +9,18 @@ Log in to ~TiddlyWeb: <$button message="tw-login" class="btn btn-info">Login</$b
 Logged in as {{$:/status/UserName}} <$button message="tw-logout" class="btn btn-warning">Logout</$button>
 </$reveal>
 
+----
+
 Host configuration: <$edit-text tiddler="$:/config/tiddlyweb/host" tag="input" default=""/>
 
 <blockquote>//for example, `$protocol$//$host$/folder`, where `$protocol$` is replaced by the protocol (typically `http` or `https`), and `$host$` by the host name//</blockquote>
 
+----
+
 <$button message="tw-server-refresh" class="btn btn-warning">Refresh</$button> to fetch changes from the server immediately
+
+----
+
+Download an offline copy of this wiki:
+
+{{$:/editions/clientserver/download-offline-button}}