Add WebAuthn UI stub

[iandunn]

We should stub out the rough code and UI for the WebAuthn front-end, so we have something concrete to build off of. This is just the rough work, refinement will be done in #193 and #194.

• this should be behind a flag so that production users don't see it until the backend is in place

WPCOM design as a reference, but it won't translate directly, and #194 will have a more refined version.

• entry screen

this might not really fit into our design. we have the account status screen instead

• naming key

• spinner while waiting on browser to receive token
  note: i don't think we should require a verification pin. it hurts the UX more than it helps security IMO

• success notice

• confirm removing key