clean up, docs, minor refactor(s) 🧹

[jessepeterson]

Various clean-up, documentation improvements, and a couple of (mostly minor) refactors:

• Make the logging keys consistent by introducing a new package called logkeys (similar to NanoCMD). Resolves Consistent logging key names #12
• Flesh out OpenAPI docs (temporary link). Resolves OpenAPI docs #4
• For the DDM endpoints store the Enrollment ID on the context and log it with all follow-on context loggers.
• Move most of the storage interfaces from the http/api package into the storage package. Resolves Move storage interfaces to storage package #3
• Make non-optional initializer values panic() for HTTP Handlers and various New() functions.
• Misc test additions and improvements.
• Misc documentation improvements (most exported symbols now have GoDoc comments).
• Refactor ddm.*Builder interfaces.
• Refactor the notifier package to abstract the DeclarativeManagement MDM command sending (notifying):
  • Import and use NanoCMD's "foss" command enqueuer