Add [x] to close site notifications

[ericholscher]

I've had a few users who have been confused by the inability to close our in-site notifications. Currently you can click on any of the links to close them, but we should also add an [x] in the corner to close them, as that's the standard UX for this.

[stsewd]

Same as readthedocs/readthedocs.org#6601, feel free to close the other one, since this one is more specific.

[agjohnson]

I won't address this in our current templates. Our new templates will already have this though.

[ericholscher]

<3