Popover/tooltip arrow class names are not consistent

Created by: MartijnCuppens

For the arrow inside of the popover, the .arrow class is used, while .arrow isn't a standalone component. We don't do that anywhere else in our css. A better name would be something like .popover-arrow. This will be something we can look in to for v5, since this is a breaking change.

Edit: We should do the same with the tooltips.