add user Configuration service for managing revisor schemas

[danijelvukoje]

Adds a Configuration service to elephant.user for managing the revisor
schemas. Schemas are grouped into config generations that are registered and
switched atomically, following the generation pattern from the
elephant-repository and elephant-distribution services:

• RegisterConfigGeneration — idempotent registration (same schema set
  returns the existing generation); optional immediate activation
• ActivateConfigGeneration — atomic switch of the active generation
• GetActiveConfigGeneration — long-poll on the active generation by known id
• ListConfigGenerations — paginated history
• GetSchema — single stored schema, active version when no version is given
• GetDeprecations / UpdateDeprecation — deprecation enforcement toggles

Each schema carries a SchemaUsage (SETTINGS or MESSAGES accepted)
declaring what it validates.

Server implementation: ttab/elephant-user#65