Feature/51 restore mail notifier

[inkhey]

Restore mail notifier from tracim v1.

• restore mail notifier for both new user notification and content_update notification
• refactor userlib with new create_user method (old one is renamed "create_minimal_user"
• do not allow anymore creating user without email (to avoid integrity error)
• disable notification of new content in both fixture and user command default behaviour.
• use ini file for tests configuration (to avoid trouble with template path in tests)
• update travis test with mailhog docker
• Add functional Test with MailHog to verify if mail system work correctly

TODO:

• Fix template assets and action url path ( those public url should be served by frontend or backend ?) see Fix url in mail template #84
• Async and Daemon support (now only sync mode is working) (Done with rq)
• Remove test code from view (not needed anymore)
• Think about a better mail Hog integration ?

closes #51

[coveralls]

Pull Request Test Coverage Report for Build 186

• 483 of 559 (86.4%) changed or added relevant lines in 17 files are covered.
• 54 unchanged lines in 6 files lost coverage.
• Overall coverage increased (+1.6%) to 76.414%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tracim/lib/mail_notifier/utils.py	12	13	92.31%
tracim/lib/core/user.py	17	19	89.47%
tracim/config.py	25	27	92.59%
tracim/tests/library/test_content_api.py	32	34	94.12%
tracim/command/user.py	2	8	25.0%
tracim/lib/mail_notifier/sender.py	53	61	86.89%
tracim/lib/mail_notifier/notifier.py	144	199	72.36%

Files with Coverage Reduction	New Missed Lines	%
tracim/command/user.py	1	38.38%
tracim/command/webdav.py	4	66.67%
tracim/init.py	4	91.84%
tracim/lib/webdav/middlewares.py	12	16.67%
tracim/lib/webdav/init.py	16	55.26%
tracim/lib/utils/request.py	17	69.23%

Totals
Change from base Build 102:	1.6%
Covered Lines:	4403
Relevant Lines:	5762

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💛 - Coveralls