Multi-select improvements

It took a chat with (albeit very helpful) support to understand that you can actually multi-select submissions in the dashboard.

Needing to hover over a non-existent element to the left of the upvote button to show the checkbox is pretty poor UX IMO and I'm sure a feature lots of users miss. There's plenty of space to always show the checkboxes in the table so I propse that they're always visible.

Also, having a "Select All" button would be useful, as would keyboard navigation (Linear are great at this).