web: Add .filter-unimportant and .unimportant CSS classes

These are used when you have a bunch of things (like rows in a table)
some of which are unimportant, and there's an optional filter
to hide them. .unimportant gets set on the thing (like the row)
and .filter-unimportant gets set on an ancestor, when you want
to hide the unimportant things.
web: Use CSS .unimportant class to implement the container filter

This is much cleaner, and separates out display from updating logic.
web: Cleaner logic for updating container and image rows

This is in lieu of using angularjs. Don't replace whole rows, so we
don't break state in those rows unnecessarily. In addition, sort the
new rows we bring in alphabetically. We'll do more complex sorting
later.