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
This `something="on"` idea is sort of what I was going for with `undismissable`. I was hoping we'd default one and make the other value-less. This means you end up with `undismissable` though which is clearly not a great name.

In this case it seems like we want `dismiss` to default to `on` and `remember` to default to `off`. It kind of feels like we want three states for `dismiss` though. I do like `remember` more than `persist` (what is persisting, the banner or the user's choice?).

I think your second group should be this:
```html
<mkt-banner dismiss="on" remember="on">I'll remember you closed me.</mkt-banner>
<mkt-banner remember="on">I'll remember you closed me.</mkt-banner>
```

But I think this is nicest:
```html
<mkt-banner dismiss="off">You can't close me (and that can't be remembered).</mkt-banner>
<mkt-banner dismiss="remember">I'll remember I was closed.</mkt-banner>
<mkt-banner dismiss="on">I'll come back.</mkt-banner>
<mkt-banner>I'll come back too since I'm the default.</mkt-banner>
```