Disable mic button if getUserMedia not available in browser

Issue #92.

src/components/Chat.vue

@@ -163,6 +163,7 @@
             elevation="0"
             v-blur
             style="z-index: 10"
+            :disabled="!canRecordAudio"
             v-longTap:250="[showRecordingUI, startRecording]"
           >
             <v-icon :color="showRecorder ? 'white' : 'black'">mic</v-icon>
@@ -517,6 +518,9 @@ export default {
       }
       return "top:" + top + "px;left:" + left + "px";
     },
+    canRecordAudio() {
+      return util.browserCanRecordAudio();
+    }
   },
 
   watch: {

src/components/VoiceRecorder.vue

@@ -139,6 +139,7 @@ const State = {
 };
 import util from "../plugins/utils";
 import VoiceRecorderLock from "./VoiceRecorderLock";
+require('md-gum-polyfill');
 
 export default {
   name: "VoiceRecorder",