Extract session managment to bespoke meta wrapper
Extract session managment to bespoke meta wrapper

Add meta wrapper for braid universal middleware (which includes
request logging and session handling).

NB: By wrapping all handlers in the universal middleware, session
encoding/decoding now happens earlier in the middleware, as does
general cookie processing.

NB: Session handling is now present universally.  Previously it was
present in almost every handler's enclosing middleware _except_ the
`public-handler`.  There is no problem with attempting to decode a
session from an anonymous user making a request to this handler and it
has the additional benefit of allow greater insight into site usage.

rafd/braid,rafd/braid,braidchat/braid,braidchat/braid