Fix accessibility issues related to the new dynamic participants table

There may be some accessibility/usability issues that need to be fixed with the newly revamped participants table.

1. If a user activates an element by keyboard (e.g. a pagination link), then the keyboard focus gets lost when the table is reloaded. A keyboard user would have to navigate again from the start of the region.
2. I wonder if the column sort links and the show/hide column links should be designated with a button role since they don't really link to another page/area, but instead toggle the display state of the table. (This one probably applies to stables as well)
3. From MDL-67915, clicking on show column icons still announces the shown column as collapsed. This is the same for the hide column icon.