Improve style checks and test them on travis

[jonc125]

I've not fixed issues in the datasets app because there's no point until #193 is merged. This PR should then be updated and any remaining style issues fixed.

ModellingWebLab/project_issues#43

[codecov-io]

Codecov Report

Merging #204 into master will decrease coverage by <.1%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #204     +/-   ##
=======================================
- Coverage    95.1%    95%   -0.1%     
=======================================
  Files          60     60             
  Lines        2533   2506     -27     
  Branches      268    268             
=======================================
- Hits         2409   2382     -27     
  Misses         88     88             
  Partials       36     36

Impacted Files	Coverage Δ
weblab/datasets/templatetags/datasets.py	81.8% <ø> (ø)	⬆️
weblab/datasets/forms.py	92.3% <ø> (-0.8%)	⬇️
weblab/core/models.py	100% <ø> (ø)	⬆️
weblab/experiments/urls.py	100% <ø> (ø)	⬆️
weblab/conftest.py	99.3% <100%> (ø)	⬆️
weblab/experiments/views.py	97.5% <100%> (ø)	⬆️
weblab/entities/forms.py	95.3% <100%> (ø)	⬆️
weblab/entities/repository.py	98.7% <100%> (ø)	⬆️
weblab/experiments/models.py	99% <100%> (ø)	⬆️
weblab/datasets/views.py	92.1% <100%> (-1.2%)	⬇️
... and 3 more

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[jonc125]

@d20gdx this should be relatively straightforward to review - it's just adding the style check config and then fixing all the issues it threw up. Do ask if anything is unclear!