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
> ```html
> <mkt-banner>I'll come back next time I'm put on the page.</mkt-banner>
> ```

what does this one do?

I like the [`autocomplete` pattern for `<input>`s](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/Input). how about we follow a similar pattern? (the ones grouped together have the same behaviour [i.e., `remember="on"` is the default for the first one].)

```html
<mkt-banner dismiss="off" remember="on">You can't close me!</mkt-banner>
<mkt-banner dismiss="off">You can't close me!</mkt-banner>

<mkt-banner dismiss="on" remember="on">I'll remember you closed me.</mkt-banner>
<mkt-banner>I'll remember you closed me.</mkt-banner>

<mkt-banner remember="off">I'll come back next time I'm put on the page.</mkt-banner>
<mkt-banner>I'll come back next time I'm put on the page.</mkt-banner>
```

let me know what you think! :sake: