Switch to official Flask-Login 0.3.0

PR for unicode cookies got accepted.
Point to a commit of official Flask-Login.
New Flask-Login is not on PyPi yet so point to GitHub.
Merge pull request #909 from PyBossa/issue-switch-official-flask-login

Switch to official Flask-Login 0.3.0
Use Flask-SQLAlchemy 2.0
Use scoped_session for slave session
Remove unused session maker
Test Slack.
Use latest version.
New strings.
New strings translated.
Fixes for the translations
Translation fixes.
Use latest version
Improvements to translations.
Update translations.
Fix a typo
Fixes to translations
Fixes for translations
Fix translations.
Improve i18n.
French updates.