Refactor MapExplorer to be Redux-friendly

[shaunanoordin]

Closes #104

The Map Explorer was originally built on (a very convoluted implementation of) the basic React patterns. As the project has progressed however, we've embraced the new Redux-React patterns which makes storing data (and communicating data between components) much more easier.

This refactor attempts to:

• Make the MapExplorer code easier to digest by anybody who isn't the madman who wrote it in the first place (Yes, @shaunanoordin)
• Make the MapExplorer component(s) more in line with the React-Redux patterns used by other components in the project

I'm still in the process of making the code lighter & easier, so shout if you have any suggestions.

[simoneduca]

@shaunanoordin LGTM, but I've got a suggestion. In ES6, var is the least informative way of declaring a variable, and let or const should be preferred when possible. Here: https://github.com/zooniverse/wildcam-gorongosa-education/blob/mapexplorer-redux-refactor/src/containers/MapExplorer.jsx#L141, as far I can see, both legend and div are not reassigned, so I think they should be const instead of var.

[shaunanoordin]

Thanks for the helpful feedback, @simoneduca ! I've gone through the Map Explorer code and improved the use of const and let across a few components. This PR is now ready for review!

[simoneduca]

👍