Retain Markers and Layers Whenever Leaving and Returning to Map Page

[ThorntonMatthewD]

Fixes #25

Summary

Fixes an issue where whenever a user would go to the About page from the Map page and then back:

• Any markers that were on the map were absent
• Hovering over the layers control produced a shriveled icon- all layer options were gone
• Users would need to refresh the page to see map data once again

Testing

1. Enable some map layers
2. Go to the About page
3. Navigate back to the map page
   1. All of your active layers should be represented in:
      1. The markers the appear on the map
      2. The query parameters in the URL
      3. The layer control should have the checkboxes for these layers still checked (and the unchecked ones should appear also)
      4. The maintainer info in visible in that control as well
4. Flip back and forth between the Map and About pages to ensure that performance does not degrade

Bonus:

1. Take fuite for a spin to ensure there aren't any glaring memory leaks pertaining to step 4 above.

   npx fuite http://localhost:4173/open-map-data-multi-layers-demo/\?lat\=34.84240835693139\&lng\=-82.4626922607422\&zoom\=12\&maps\=city-halls,breweries --iterations 10

2. Compare your results with the live version at https://hackgvl.github.io/open-map-data-multi-layers-demo to make sure no new issues stem from these changes.

Thank you!

[ThorntonMatthewD]

Thank you so much for fixing this bug, this is amazing!

@oliviasculley Thank you for taking a look at this!