inveniocfg: fix --drop-tables command

* Solves problems with droping tables using SQLAlchemy models
  (closes #1283).

Co-authored-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Flask: integration of legacy unit tests

* Adds new `@with_app_context(...)` decorator for ensuring application
  context in functions called from command line before they will be
  ported to inveniomanage script.

* DEPRECATED `flaskshell` will be removed and replaced with
  with_app_context decorator or application factory usage.

* Fixes UserInfo object that was returning uid == 0 instead of -1 for
  guest users (webuser_unit_tests.py).

* TODO check inveniocfg_upgrader for upgrades from non-Flask based
  installations.
travis: initial configuration release