Handle enums that are not all-caps better

[yifanmai]

We have a number of persisted models with enum types in which the string representations of the enums are camel-cased. Courier only supports all-caps enums, so these enums cannot be migrated to Courier.

Possible solutions:

• Allow enum symbols to be camel-cased, rather than enforcing all enums to be all-caps.
• Allow alias values for enum symbols i.e. allow the string representation of the enum symbols to be specified.

[kchen-coursera]

Actually, courier seems to handle non all-caps enums just fine in my experience. It's just a matter of inconsistency with our naming convention