Give the Jitsi widget an icon to help with discovery

We think users will be expecting to follow a series of icons into their conference, so we'll remind them here of what is going on with a giant icon.

This does not change depending on the call type because the jitsi widget doesn't understand the call type anyways - it's always a video conference.

src/vector/jitsi/index.html

@@ -9,7 +9,7 @@
 <div id="joinButtonContainer">
     <div class="joinConferenceFloating">
         <div class="joinConferencePrompt">
-            <!-- TODO: i18n -->
+            <span class="icon"><!-- managed by CSS --></span>
             <h2>Jitsi Video Conference</h2>
             <div id="widgetActionContainer">
                 <button type="button" id="joinButton">Join Conference</button>