Change tile server for berlin images

[tordans]

This is a follow up to #480.

It is awesome to have the Berlin aerial photography right in iD now.
However, the image server by the city seems to be a bit slow.

Speed comparison

City	OKFN
~6s waiting, URL	~37ms waiting, URL

The OKFN source is based on https://tiles.codefor.de/ / https://codefor.de/projekte/2018-04-09-fis-broker.html

Note: The 2011 data are not available with tiles.codefor, so I left them be.

Todos

• Can someone validate that this changes will work?
• I wonder if we need the "required": true attribute for the attribution part, other datasets have it; what does it do?

[bhousel]

Can someone validate that this changes will work?

These changes look good to me - the best way to test is to open iD and paste the url into the "custom" . I tried this for the 2017 layer and it does load much faster.

I wonder if we need the "required": true attribute for the attribution part, other datasets have it; what does it do?

I think it just means that an editor is required to show the attribution line.

[bhousel]

I switched from {z} to {zoom}, removed the available_projections (not applicable for tms sources) and merged.