[Feature] Move LoginButton to be a DropDownButton #63

Describe the problem this feature would solve

Now that our min target is 1809, we can make use of DropDownButton. We should use that as a base class for the LoginButton vs. our custom button scenario/flyout.

We do need to think about how we adapt the style of a DropDownButton to encompass the user login and if we keep the default chevron, etc...