Add download button, number modal, & video modal

Add the download button for non-Australis folk ([Bug 990645](https://bugzilla.mozilla.org/show_bug.cgi?id=990645))
Build out the "What is this number?" modal ([Bug 990657](https://bugzilla.mozilla.org/show_bug.cgi?id=990657))
Add video modal with placeholder video ([Bug 990647](https://bugzilla.mozilla.org/show_bug.cgi?id=990647))