Ability to apply sort on mobile in portrait mode

[simonw]

Missed this in #189... on mobile in portrait mode we hide the column headers, which means you can't click them to sort! You can sort in landscape mode at least.

Need to come up with an alternative sort UI for portrait on mobile.

[simonw]

I may do this by adding select boxes for _sort and _sort_desc to the filters UI. This would allow sorting in mobile portrait mode but would also ensure that the existing sort order is persisted if the user edits the current filters (right now sort resets when filters are applied).

[simonw]

Since you can't apply _sort and _sort_desc at the same time, maybe just one select box for picking the column to sort by and a boolean checkbox for "sort descending" - which then redirects to the _sort_desc= URL variant.

[simonw]

[simonw]

Demo: https://fivethirtyeight.datasettes.com/fivethirtyeight-2628db9/twitter-ratio%2Fsenators?_sort_desc=replies&text__contains=bipartisan