Fix Slack call overlay on Awesome WM

If you use awesome and make a Slack call, you'll constantly have an overlay window pop up taking over half of your screen.

To fix this, add the following rule to your awful.rules.rules section in your rc.lua:

Copyawful.rules.rules = {
  -- ...
  { rule = { name = "Slack Call Minipanel" }, properties = { floating = true, ontop = true } },
}