alertlink: refactor to use composition

[JonasBa]

Refactor AlertLink to use the actual alert component and improve types. There are quite a few gotchas with this component and its types, and I expect there to be some unwanted behavior out there. This PR highlights all of those issues (see the number of todo comments) and addresses some.

Before:

After

Some, but not all issues include:

• To wide of a prop definition - you could pass to, href and onClick at the same time, not really knowing which one is used?
• Unless explicitly set to false, openInNewTab prop would result in the link always being open in a new tab due to the default initializer inside
• Visual discrepancy from the actual alert component
• Default margin being set and overriden in some cases. This has been updated to leverage a known pattern that @michellewzhang introduced for the Alert component
• Lack of tests - we now test for the some basic scenarios
• if only onClick is set, then a default value of to='/' is set, meaning event propagation likely has to be handled inside the onClick handler, else the navigation will still fire.