Change how room avatars are passed, fix friend avatars not loading

The problem lies in how media URLs work, in this case it the old
NeoChatRoom::avatarMediaId could pass a mxc url *or* a path that can
be put into root.connection.makeMediaUrl. So normal rooms with avatars
loaded, but never friends because room members gave the mxc URL.

Instead, change everything to use avatarMediaUrl which corrects this
issue by always passing a mxc URL to QML. This also removes the need to
call makeMediaUrl.

Fixes #675

src/models/roomlistmodel.cpp

@@ -212,7 +212,7 @@ QVariant RoomListModel::data(const QModelIndex &index, int role) const
         return room->displayName().toHtmlEscaped();
     }
     if (role == AvatarRole) {
-        return room->avatarMediaId();
+        return room->avatarMediaUrl();
     }
     if (role == CanonicalAliasRole) {
         return room->canonicalAlias();

src/models/roomtreemodel.cpp

@@ -324,7 +324,7 @@ QVariant RoomTreeModel::data(const QModelIndex &index, int role) const
         return room->displayName();
     }
     if (role == AvatarRole) {
-        return room->avatarMediaId();
+        return room->avatarMediaUrl();
     }
     if (role == CanonicalAliasRole) {
         return room->canonicalAlias();