Rename CTA rail data constant SERVICES to COLORS

Colors are soon going to be associated with routes, making the name
"services" inappropriate.
Major refactoring for all CTA rail responses

Cleans up all responses to use consistent output formats for lists and
references (i.e. "has-many" and "has-one" relationships), shorter key
names, and key-to-value mappings only for the API side for quick lookup.