Firefox Accounts migration banner (bug 1052864)

https://bugzilla.mozilla.org/show_bug.cgi?id=1052864

This adds custom elements for the site banner (`<mkt-banner>`) and login (`<mkt-login>`).

r? @cvan
Should `undismissable` be part of `dismiss`?

```html
<mkt-banner dismiss="none">You can't close me!</mkt-banner>
<mkt-banner dismiss="persist">I'll remember you closed me.</mkt-banner>
<mkt-banner>I'll come back next time I'm put on the page.</mkt-banner>
```