Drop Django <=1.10 support

[grahamu]

Add namespacing to context_urls.py, remove patterns use.
Remove compat.py.
Add "PINAX_TEAMS" AppConf prefix for all settings.
Expand documentation.
Fix migration on_delete for null=True fields.
Update test configuration

[codecov]

Codecov Report

Merging #57 into master will decrease coverage by 1.46%.
The diff coverage is 27.27%.

@@            Coverage Diff            @@
##           master     #57      +/-   ##
=========================================
- Coverage   49.17%   47.7%   -1.47%     
=========================================
  Files          19      16       -3     
  Lines         728     698      -30     
  Branches       77      76       -1     
=========================================
- Hits          358     333      -25     
+ Misses        363     359       -4     
+ Partials        7       6       -1

Flag	Coverage Δ
#py27dj110	?
#py27dj111	47.7% <27.27%> (-1.2%)	⬇️
#py27dj18	?
#py34dj110	?
#py34dj111	47.7% <27.27%> (-1.2%)	⬇️
#py34dj18	?
#py34dj20	47.7% <27.27%> (-1.2%)	⬇️
#py35dj110	?
#py35dj111	47.7% <27.27%> (-1.2%)	⬇️
#py35dj18	?
#py35dj20	47.7% <27.27%> (-1.2%)	⬇️
#py36dj111	47.7% <27.27%> (-1.2%)	⬇️
#py36dj20	47.7% <27.27%> (-1.2%)	⬇️

Impacted Files	Coverage Δ
pinax/teams/forms.py	42.85% <0%> (ø)	⬆️
pinax/teams/middleware.py	0% <0%> (ø)	⬆️
pinax/teams/context_urls.py	0% <0%> (ø)	⬆️
pinax/teams/hooks.py	80.95% <100%> (ø)	⬆️
pinax/teams/models.py	67.74% <100%> (ø)	⬆️
pinax/teams/templatetags/__init__.py

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[grahamu]

@paltman and/or @jacobwegner I'd appreciate your eyes on this PR bringing pinax-teams up to date with our latest supported Django/Python configurations. I believe this will resolve PRs #36 and #55. I'm also a bit leery of my updated requirement for django-reversion in light of #33 and #56 because I do not understand Jake's comment "breaks semantic versioning".

[paltman]

I'd be in favor of dropping django-reversion and letting the site builder decide how they want to version things, assuming that is possible to do with external packages.

[grahamu]

@paltman and @jacobwegner after weighing pros and cons about removing django-reversion, I have decided to keep that requirement in place for the initial Django v2.0-compatible release. This gives existing users who rely on django-reversion a path forward with Django 2.0.

Later, after a bit of testing to find user-installed alternatives, we should remove django-reversion.