Put api keys directly in js layer definitions

[AntonKhorev]

Currently map layer API keys have one extra level of indirection:

• keys are defined in the settings
• osm.js.erb reads them from the settings and writes them to OSM.THUNDERFOREST_KEY and OSM.TRACESTRACK_KEY
• layers.yml contain names of OSM properties to check for keys

Instead of this osm.js.erb could write keys directly to layer definitions, skipping OSM.THUNDERFOREST_KEY and OSM.TRACESTRACK_KEY. layers.yml could contain the corresponding settings names. The advantage is that Thunderforest and Tracestrack keys are no longer special. A new key can be added just by editing settings.yml and layers.yml. Also it's possible to skip layer definitions if they require a key and that key is missing.

[tomhughes]

I'm sure this could be improved, but a massive multi line block of ruby embedded is a js file is definitely not the answer to my mind at least.

[AntonKhorev]

I moved that ruby code to a module, but now there's no dependency on it. If it's changed, osm.js won't be recompiled.