More layout fixes for comments
Fixed z-index hierarchy issues;
Some cleanup
Make sure sheet_header_top_container is not promoted to a different layer so the menu doesn't open underneath the items list
Made dataType: 'json' the default
Fixed user field validation and serialization on SheetItemCommentBaseSerializer by implementing UUIDRelatedField
Implemented comment creation flow;
Started adding Scrolled to items_comments_container
Close comments panel on any change to items list state
Make sure clearabletextinput's background color is transparent no matter what
Some cleanup
Fixed errors in file export view when called without a mode or model different from sheet
Hover state for items_comments_close