[399] Fix build by integrating with Mirador 3 + Image Tools Plugin

[jsavell]

This PR resolves #399 by manually building Mirador 3 with grunt-webpack to enable packaging the image tools plugin and potentially other future plugins.

This required the implementation of a wrapper object to provide access to Mirador's state while in the context of the angularjs app.

The destroy method in particular is needed to properly unmount the Mirador instance when the contentViewer directive is unloaded by angularjs. This is necessary to prevent rendering issues if a user navigates to another Mirador based discovery view.

This solution appears to be working without issue, but I couldn't find any documented examples of dynamically generating and destroying Mirador instances. The 'proper' use of Mirador 3 seems to be loading one instance and then using redux actions to update it as needed.

We are using redux actions to manipulate the state of the Mirador instance, but due to angularjs's dom element manipulation, we can't sanely preserve a single instance across user navigation.

This wrapper does provide a good starting point for a more generalized component that's not specific to SAGE.

[coveralls]

Coverage decreased (-0.07%) to 45.567% when pulling 023796c on 399-mirador3-webpack into 280ec1d on master.

[ghost]

Looks good for code review. Can we get this deployed to dev for testing?

[jsavell]

I switched back to using Mirador.viewer to generate the Mirador instance. I had needed to bypass that at one point for an earlier approach that didn't pan out.