Dark mode adjustments for popover

Add new popover classes from @nextcloud/vue's Popover and PopoverMenu*
components. This fixes issues where the menu icons were not inverted.

Adjusted border for both the avatar popover menu and also actions
popover menu to make them easier to distinguish from the background.

Now using border color for the popover arrow to make it more visible
outside the border.

Signed-off-by: Vincent Petry <vincent@nextcloud.com>