This commit adds support for the LN checkout flow, and the Purple landing page: 1. It adds a "learn more" button on the Damus Purple view, where the user can learn more 2. It adds new `damus:purple` urls to enable the LN checkout flow Signed-off-by: Daniel D’Aquino <daniel@daquino.me> Reviewed-by: William Casarin <jb55@jb55.com> Signed-off-by: William Casarin <jb55@jb55.com>