Refactor flask-zodb into lhs.frontend.conmgr
DB: remove dbext_factory
requirements: pin zodbpickle at 0.5.0 for now
conmgr: doom the default transaction, abort on close

This makes the transaction unusable, so we're forced to use nice, short-
running transactions instead.
Frontend: remove util.readonly (redundant)
conmgr: docs
docs: add lhs.frontend.conmgr