water-api: Delete Visualization Endpoint

[brettpalmberg]

Add an endpoint to the water-api to delete visualization. This will be available from the user interface here (delete button shown below):

The request will be sent to DELETE /visualizations/:visualization_slug.

For example, deleting visualization alum-creek-dam-profile-chart would require:

DELETE /visualizations/alum-creek-dam-profile-chart

[brettpalmberg]

This is working great. Thanks @adamscarberry. Closing now.