Add Early Warning event header

[jmfee-usgs]

When the preferred origin product's source is ew (early warning), show a banner with the following markup:

This event information is from the
<a href="https://earthquake.usgs.gov/research/earlywarning/">Earthquake Early Warning</a>
project and is <strong>preliminary</strong>.
Please check this page again in a few minutes for more information.

Similar to the banner shown for scenarios https://github.com/usgs/earthquake-eventpages/issues/297