Upgrade Django (Django-1.10 no longer supported)

[ghost]

Django 1.10 is now no longer supported, this PR upgrades modoboa to Django 1.11 (the last version of Django to support Python 2). This depends on modoboa/modoboa#1333

I've split this into several patches to make it easier to follow:

• 87f2297: make all the travis.yml settings the same across modoboa projects.

• bbff0e0: updates for django 1.11 support

  • reverse and reverse_lazy have moved to django.urls

  • MIDDLEWARE_CLASSES is replaced by MIDDLEWARE

  • RadioChoiceInput widget has been removed, replace with HorizontalRadioSelect

• 3c09b1d: random code clean ups

  • whitespace fixes

  • added missing r"" prefixes where required

  • replace unicode with six.text_type

  • {} is more efficient than dict()

• b802be1: updated test_project, synced with a fresh deployment using Django 1.11.8 and updated settings.py so the tests environment matches a fresh deployment.

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As this depends on modoboa/modoboa#1333 tests on travis will fail until it's merged, I've created a special branch (with altered travis.yml) to prove the tests work upgrade-django-TESTS

[codecov[bot]]

Codecov Report

Merging #136 into master will increase coverage by 0.01%. The diff coverage is 88.23%.

@@            Coverage Diff             @@
##           master     #136      +/-   ##
==========================================
+ Coverage   73.98%   73.99%   +0.01%     
==========================================
  Files          22       22              
  Lines        2010     2011       +1     
==========================================
+ Hits         1487     1488       +1     
  Misses        523      523

Impacted Files	Coverage Δ
modoboa_webmail/forms.py	76.6% <ø> (ø)	:arrow_up:
modoboa_webmail/lib/sendmail.py	83.87% <100%> (ø)	:arrow_up:
modoboa_webmail/lib/imapemail.py	65.93% <100%> (ø)	:arrow_up:
modoboa_webmail/templatetags/webmail_tags.py	72.36% <100%> (+0.36%)	:arrow_up:
modoboa_webmail/handlers.py	60% <100%> (ø)	:arrow_up:
modoboa_webmail/tests/test_views.py	100% <100%> (ø)	:arrow_up:
modoboa_webmail/lib/fetch_parser.py	95.83% <100%> (ø)	:arrow_up:
modoboa_webmail/modo_extension.py	100% <100%> (ø)	:arrow_up:
modoboa_webmail/views.py	65.59% <66.66%> (ø)	:arrow_up:
modoboa_webmail/lib/imaputils.py	59.95% <80%> (ø)	:arrow_up:

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