Loading time improvements

[Bo-Duke]

All loadings of the app have been significantly reduced :

• Limited number of requests made on each actions, only make request if necessary for the next UI change
• GeoJSON files are now fetched separetely from the settings and the list of layers, meaning the UI is usable while the files are still fetched
• Activating / deactivating a layer changes the state of the switch instantly, the timing of side-effects don't affect it
• MapContext fetches settings itself, meaning the UI can render before the request is finished

Loading indicators on some UI elements :

• Map is rendered as soon as possible, the sidebar has "skeleton" layers while waiting for the settings to be fetched
• When a layer is activated but its GeoJSON is still loading, the switch displays a loader
• Details page has a spinner

Results :
Clicking on "streams" from the homepage

Before : 6 seconds until usable UI and map, with streams displayed
Now : < 1 second until usable UI and map, streams are displayed at 6 seconds

Clicking on a river on the map

Before : 6 seconds until details are shown
Now : 0.9 second until details are shown

Todo:

• Activating layers in the url on first load