Unify room welcome header handling

Also, add the "WelcomeHeaderChannel" component

src/components/RoomInfo.vue

@@ -152,7 +152,7 @@
               />
             </v-list-item-avatar>
             <v-list-item-content class="pb-0">
-              {{ messageRetention }}
+              {{ messageRetentionDisplay }}
             </v-list-item-content>
 
             <v-list-item-action class="pb-0 mb-0">
@@ -194,7 +194,7 @@
           <div class="option-title">{{ $t('room_info.message_retention') }}</div>
           <div class="option-text">{{ $t('room_info.message_retention_info') }}</div>
         </div>
-        <div class="text-right">{{ messageRetention }}</div>
+        <div class="text-right">{{ messageRetentionDisplay }}</div>
         <v-btn v-on:click="showMessageRetentionDialog = true" v-if="canChangeRetentionPolicy" icon size="x-small"><v-icon color="black">edit</v-icon></v-btn>
       </v-card-text>
     </v-card>
@@ -336,7 +336,6 @@ export default {
       showLeaveConfirmation: false,
       showPurgeConfirmation: false,
       showMessageRetentionDialog: false,
-      messageRetention: "",
       buildVersion: "",
       updatingJoinRule: false, // Flag if we are processing update curerntly
       joinRules: [
@@ -446,7 +445,7 @@ export default {
   methods: {
     onMessageRetention(retention){
       const retentionPeriodsFound = this.retentionPeriods.find(rp => rp.value===retention)
-      this.messageRetention = retentionPeriodsFound.text
+      this.messageRetentionDisplay = retentionPeriodsFound.text
       },
     onListItemClick(member) {
       this.activeMember = member
@@ -457,16 +456,10 @@ export default {
         // For this room
         if (event.getType() == "m.room.member") {
           this.updateMembers();
-        } else if (event.getType() == "m.room.retention") {
-          this.updateMessageRetention(event);
         }
       }
     },
 
-    updateMessageRetention(event) {
-      this.messageRetention = this.roomMessageRetentionDisplay(event);
-    },
-
     updateMembers() {
       if (this.room) {
         const myUserId = this.$matrix.currentUserId;