Create a state dropdown

[joshsmith]

Problem

#849 added a basic state dropdown that needs to be populated with actual data.

https://github.com/code-corps/code-corps-ember/pull/849/files#diff-fadc9970a9b163a9321d697cbdd42755R63

[joshsmith]

https://en.wikipedia.org/wiki/List_of_U.S._state_abbreviations#Table

We'll need the name and USPS code for every place that has a non-obsolete two-digit USPS code.

[joshsmith]

The various groups will be placed into optgroups.