This gives us our standard logging and passes around errors with
impart.HTTPError.

Ref T705

this changes the helper IsJSON to take a request instead of a string,
allowing to check multiple headers. In this case both Content-Type and
Accept.

Ref T493

This adds a new ReqLog() func to the Apper interface that'll return the
log message for incoming requests.

Ref T649

Previously, we just included the value of `ul`, which is a func. This
now calls `ul()` and logs that value.

(for private instances)

Ref T576

This verifies that a user is authenticated before getting to the actual
handler on API endpoints where a user is reading content.

Ref T576

This is the start of all changes needed to support entirely private
instances, where all blogs are only visible to other authenticated users
on an instance (ref T576). It begins by changing how Handler methods check an
endpoint's permissions.

- Renames UserLevelLEVEL consts to UserLevelLEVELType
- Adds UserLevelLEVEL funcs with same names as previous consts. Each
  returns a UserLevel
- Adds a new UserLevelReader that restricts access based on app
  configuration. This is now used on collections and posts.
- Changes routing a bit so static files are always accessible

Ref T613

- Adds a new interface, Apper, that enables loading and persisting
  instance-level data in new ways
- Converts some initialization funcs to methods
- Exports funcs and methods needed for intialization
- In general, moves a ton of stuff around

Overall, this should maintain all existing functionality, but with the
ability to now better manage a WF instance.

Ref T613

Start of T538