refactor: eliminate unnecessary calls to /ds/get

[chriswhong]

Closes #266

We had api calls spread across DatasetRoutes, DatasetWrapper, and DatasetPreviewPage, resulting in 3 separate calls to the same endpoint (/ds/get) when a preview route was loaded.

• We have separate state for dataset and preview but they are both populated by the same data, so this refactors the reducers for both to listen for the same action.
• Calls happening in DatasetRoutes are moved down into DatasetWrapper so they are all happening in the same place, leaving DatasetRoutes to just be a router, not a router and a data-getter.

[chriswhong]

I pushed a commit which:

• restores dispatching fetchDsPreview in DatasetPreviewPage
• restores API_PREVIEW_XXX reducers in dsPreviewState
• removes loadDataset from DatasetWrapper

This way, we have everything only fetching data when it needs to:

• DatasetWrapper gets the header, workflow, and runlog. These are what is needed for all dataset routes. (ideally we would be able to determine in the call to header whether there is a workflow and run activity so these calls can be moved down to their respective route components, but we can do that later)
• DatasetPreviewPage fetches the preview
• DatasetComponents fetches whatever dataset version is specified.

I think #3 you mentioned above is a separate issue, we should make sure preview is updated after a commit.