add simple CircularProgress component when loading

[nichhk]

Partially fixes #1284

Major changes:

• Get rid of isVisLoading from redux state since it's not used

• Get rid of redux-saga takeEvery for GET_DATA_REQUEST, since we're already implementing the side effect outside of redux-saga

• set store.data.isMapLoading every time we get new data, and unset it once we successfully retrieve the new data

• create a CircularProgress component when store.data.isMapLoading is true. This is just a temporary solution until we have a design for an actual loading indicator.

  • Up to date with dev branch
  • Branch name follows guidelines
  • All PR Status checks are successful
  • Peer reviewed and approved

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[nichhk]

Thanks for the review Shannan!