PMO - Normalize server id keys

[andrejewski]

Currently, the PMO server returns restaurant, state, and city data objects. Each has its own id key (_id, short, and name respectively) which tripped a few of us up running through the guide. It's good to know you can have arbitrary id keys but not something to prioritize in this guide because the generator does the work of making the files.

• The server should be updated to normalize on _id (or even better id so the generator steps are even easier because id is the default).
• The PMO guide should be updated to match the new server ids.