Allow users to select comparison map from share page

[LanesGood]

A proposed workflow to enable comparison without needing to first visit /public-maps:

As a user, I can:

1. Load share map
2. Click a "compare" option, which exposes a dropdown, drawer or modal
3. Within dropdown/drawer/modal, I can see all shares visible on the /public-maps page
4. Selecting on one of the other shares navigates me to the compare tool

This would require loading all of the public shares into the shares route, which I could imagine causing some performance concerns. It also requires refactoring the /compare page to accept just one uuid and then have the second set within the page

cc @vgeorge @geohacker