Add a constants src file

[manthey]

Currently, we have constant values in both src/events.js and src/actions.js (and possibly elsewhere). It would be handy if there was a src/constants.js file that exported all of these in a single namespace (geo.constants.*).

This would also be an appropriate place to add additional constants to replace some internal special values, such as the use of null as the gcs to mean the map's display gcs. See the conversation in PR #654 for more background on this.