Variant sets and their sizes (V205).

One tab per variant set (?set=<uuid>; none is the Default). A set says which derived files a library's uploads get: its sizes (dimensions), and whether sizes and zoomable tiles are made automatically. The standard sizes are listed first and cannot be deleted. A new set starts with a copy of the Default's standard sizes. Changing a set or a size bumps the set's revision and the reconciler brings its files up to date; "Check every file" does that without a change (a size a file never got is made).