update flask-mail to current git version (fixed commit), set 'Sender' header in outgoing mail and smtp envelope

(cherry picked from commit a9c8b71f7a7989ab21caf8c816d1f00ac62f9f21)

Conflicts:
	abilian/core/extensions.py
celery: install sentry handlers
tox: generate junit xml
repository tests: cosmetic changes
repository service: remove global logger instance
session repository: create transaction directory only after "begin", not at instanciation time

Conflicts:
	abilian/services/repository/service.py
repository: fix when using sqlalchemy's subtransaction (!= nested)
fix date/datetime fields not supporting years before 1900
if CONFIG_ENVVAR is set, the file must exist