Add Alerts component

[vine77]

Resolves PDS-585

• Add Alerts component, a container component to which you can pass a collection of Alert components to display them as floating "growl" notifications.
• Add documentation with a couple examples.

[vine77]

I’m open to suggestions for other variants of this if it would be helpful to have a non-floating version or bottom-right placement option.

[mardotio]

I think this looks good. For what we need them we only need them to show up in the upper right corner, but it seems like it should be easy enough to add other positions is we ever need them.
Thanks for getting this done!