Add Importance#getName and Importance#fromName so that callers stop parsing importance names themselves

The Importance enum of the Java API only converts to and from the number an importance is stored as (0/1/2). The name a person or a client writes (low/medium/high) was therefore parsed twice, once by the change query parser and once by the REST representation factory.

This adds, to the @Unstable Importance enum:

• getName(): the lower cased name of the importance, e.g. high
• fromName(String): the importance of that name, whatever its case, or null

and makes both callers use them. No behaviour change for users.

Done along with an internal refactoring giving the ChangeClass/EntryClass schema of a change a single owner in the -api module (property names, reading, and writing over the template or replacing).