fix(Communities): fix channel popup position and field hydration

There are two regressions introduced in 080767c338 where
the `CreateChannelPopup` isn't properly centered when triggered in edit mode,
and that the channel name field isn't hydrated with channel data.

This commits fixes both bugs.

ui/app/AppLayouts/Chat/CommunityComponents/CreateChannelPopup.qml

@@ -37,6 +37,7 @@ StatusModal {
         contentComponent.channelName.text = ""
         if (isEdit) {
             header.title = qsTr("Edit #%1").arg(channel.name);
+            contentComponent.channelName.text = channel.name
         }
         contentComponent.channelName.forceActiveFocus(Qt.MouseFocusReason)
     }

ui/app/AppMain.qml

@@ -520,6 +520,7 @@ StatusAppLayout {
     Component {
         id: editChannelPopup
         CreateChannelPopup {
+            anchors.centerIn: parent
             isEdit: true
             pinnedMessagesPopupComponent: chatLayoutContainer.chatColumn.pinnedMessagesPopupComponent
             onClosed: {