Replace fetch and global pending state

[rachelekm]

This project currently uses a custom Conveyal library feature (woonerf’s fetch) as middleware to handle async logic/data fetching within action dispatches. Each call to fetch automatically dispatches another action to increment fetch state, which in turn updates a global pending state (this isLoading state functions as a global conditional for component display until load/state updates are complete). This causes issues as we implement new api calls and want to isolate the effects of a pending state to the component level (see PR #374). Switching to axios and replacing a global pending state dependent on the number of fetches will help us avoid this issue. In addition, we would be better served to replace fetch with axios for more intuitive data fetches and robust error handling.