Refactor contents of `api` package into `base` and peer subpackages

[mansam]

base subpackage should contain things like the BaseHandler (as base.Handler), context, constants, etc that are used by other handlers. Allows better organization of subresource handlers and sharing between handlers.

• base
  • context.go
  • handler.go
• application
  • handler.go
  • tags.go
  • facts.go
• reports
  • etc