[#665] Make autoupdate work on form submit
[#665] Make sure that we use the POSTed date time on a failed POST

We need to have a valid $post reference here if we did post (but we
need to check if we did post first, so that the existence of a value on
$post won't interfere with prepopulate on a GET)
[#665] Specify a container for the date/time picker

By default it goes immediately after the associated input, but in chrome
that makes picker be constrained to the same width/height of the
associated input -- awkward

Also always make the time input beside the date input even on smaller
screens. Looks better, and less awkward with the positioning of the
pickers
Fix Celerity / Gradation Vertical being too narrow

Our desired content width is 64em. Unfortunately in Celerity / Gradation
Vertical, the sidebar is counted in the 64em, making the actual content
much narrower.
[#665] Fix wrapping of labels for checkboxes/radio buttons
[#665] Replace %visually-hidden with the element-invisible() mixin
[#665] Add icon-buttons component

icon-buttons.tt is for icon-only buttons. It includes a text fallback
(necessary for older browsers / accessibility).
[#665] Tweak variable name
[#665] Crosspost component
[#665] Remove some unneeded code