R1:fec00256be1f

Allow collapsing/expanding workboard column content by clicking its header

Summary:
Reduce users' need for scrolling on smaller screens with 920px or less viewport width by using HTML5's `<details>`/`<summary>` so clicking on a workboard column header hides the content of that column, in all CSS views (mobile, tablet, desktop). Keep expanding its content by default.

On mobile and tablet devices, display an arrow in the column header box below the header text to potentially make those users aware of this…