Deploy Omeka tag v18.0.2 by default and allow Omeka-specific API key

* Exhibitions was previously set to tag v16.1
* Allow Omeka to use its own specific API key. This allows us to
more easily filter out API requests from Omeka in our monthly
statistics.
Merge pull request #39 from dpla/update-omeka-configuration

Deploy Omeka tag v18.0.2 by default and allow Omeka-specific API key