viewer: use order_by() explicitly, helps with keeping indices up-to-date

viewer: basic line chart for checks (errors, warnings, duration)

viewer: use two columns for info lists on check page for small screens too Since lists themselves switch to one column on small screens, there's enough space for both of them side by side.

viewer: get reference check in a separate method

viewer: create an index for checks to speed up "order by" a lot

viewer: missing dot.svg template

tests: shared XML namespace dictionary (for checking Atom and SVG)

tests: shortcut for attrib check (it works quite well with pytest)

viewer: show a solid color dot on check page to bring attention to the result

viewer: short summary and a link to compare on check page when makes sense