actions for new dataset creation flow & E2E tests

[b5]

requires #601, #602. Once those are landed, we should have an action that:

• accepts a Dataset argument
• constructs a "form-file" POST request to the backend with the contents of the dataset data structure
• does the POST
• redirects to the new dataset on success

This issue expects an E2E test for the flow, specifically a new test that takes the following steps:

• navigates to the collection home
• clicks new dataset
• picks a name & hits create
• tries to click commit, sees an error
• adds meta
• stages a body file
• creates a readme
• clicks structure tab
• clicks submit
• waits for navigation to end on the expected dataset name