GlAvatarsInline: Add tooltip to badge

When using GlAvatarsInline, a badge next to the avatars shows the number of remaining avatars when the total number of avatars exceeds the given maxVisible prop. Let's add a tooltip to the badge which shows the names.

We should consider the following:

1. We need to specify a prop which will be used for the tooltip content
2. We might need to add a threshold prop (either a max characters to display or max names to display so in case the of a large number of remaining avatars, the tooltip won't bloat.
3. We should also truncate the tooltip text via JS (and append ...) when the threshold from (2) is exceeded.