Refactor /dataset route

[lwrubel]

Currently the /dataset route handles logic for 5 tabs on the dataset.html template. This means it has to handle form submissions and logic for multiple operations at once and has become unwieldy for making any further changes.

Recommend we break out the tabs into separate routes and templates so logic can be handled where it makes sense. This will also make it easier to add server-side form validation (look at WTForms and Flask-WTF for example).