Improve URL Previews

only show 2 by default with expand/collapse mechanism
show all hashes again, but dedup requests
clean up hide mechanism, instead of one `x` per preview have one per group

res/css/_components.scss

@@ -201,6 +201,7 @@
 @import "./views/rooms/_GroupLayout.scss";
 @import "./views/rooms/_IRCLayout.scss";
 @import "./views/rooms/_JumpToBottomButton.scss";
+@import "./views/rooms/_LinkPreviewGroup.scss";
 @import "./views/rooms/_LinkPreviewWidget.scss";
 @import "./views/rooms/_MemberInfo.scss";
 @import "./views/rooms/_MemberList.scss";