Use Flask-SQLAlchemy 2.0
Use scoped_session for slave session
Remove unused session maker
Merge branch 'master' into issue-flask-sqlalchemy2.0

* master: (30 commits)
  Fix test for admin featured
  Missing ¡
  New compiled .mo file
  Removed translations with no appearances
  Change remaining 'aplicaciones' for 'proyectos'
  Fix typos and changed applications for projects in Spanish
  New strings to traslate.
  Use latest version.
  More i18n.
  Add i18n.
  Update i18n.
  Update i18n.
  New translations
  Compute stats always for featured projects.
  French updates.
  Improve i18n.
  Fix translations.
  Fixes for translations
  Fixes to translations
  Fix a typo
  ...
Attach a slave_session attribute to db, remove get_session
Remove use of get_session and use db.slave_session instead
Use category_id instead of category to prevent autoflushing
Explanation about flask-sqlalchemy behaviour
Simplify
Renamed function
Restrict version dependencies
Merge pull request #912 from PyBossa/issue-flask-sqlalchemy2.0

Issue flask sqlalchemy2.0