Add admin-bounds overlay

[danielfdsilva]

• Depends on Generate vector tiles for admin bounds ram-backend#182 for server side endpoints
• Depends on Add map layer selector #149 for layer selector

[olafveerman]

This looks good 👍
Made a commit with some small style tweaks to the map.

Also, I noticed a visual glitch with the toggle:

1. open the 'toggle layers' selector
2. click on a link (like Projects in the breadcrumb)

The layer selector will appear briefly on the left top of the screen.
We can log this as an issue to tackle later, feel free to merge if you want.

cc @danielfdsilva