Add map attribution

[Binnette]

As asked in issue #52, I added the default attribution on the bottom right corner of the map to comply with OSM rules.

I had to move the fab button a bit upper to avoid covering the attribution button provided by Mapbox GL.

@DoFabien Can you please review my "code" and tell me if it is fine for you?

Here are some preview:

By default the button "i" is compacted:

When you click on it, it expands and show credits:

[DoFabien]

I agree with that. Otherwise, I had thought of opening the "about" page with the attributions, integrating also the attribution of the current base map (after integrating #110 ).

We need to change the default attributions. It redirects us to https://www.mapbox.com/contribute/