Reviewing admin units

Change the two boolean variables used in computeNeedsReview to dates, so that needsReview is accurate at every point in time during the week. (hasClassChanged boolean was only updated weekly, so even if an expert were to review "now", it would still be shown on the validator until the next model run prep date, potentially days later).

Map uses this newly corrected value: "disease extent layer" is a group layer that has two layers - admin units that need review by the expert, and those that have been reviewed. The latter are not selectable from the map or the side panel table, and are styled with cross hatching - achieved by a baselayer pattern in GeoServer.

Finally, added a spinner to display on layer change loading times - separated out into a separate ViewModel.

Unit tests?