Add UI for fetching iNaturalist data

[johnbradley]

Adds a navigation bar to navigate between home(visualize), and a new 'Fetch Data' page. The 'Fetch Data' page shows a prompt for users to enter a iNaturalist user id. After the user clicks the "Fetch Observations" button, the user will be shown a subset of the observations found and a "Download CSV" button to download all observations as a CSV file. The expectation is users will add additional numeric columns to the CSV, then upload this file on the home page for visualization.

Part of issue #24

[johnbradley]

New navigation added to the top of the home screen:

Fetch data page:

Observations on fetch data page: