Allow user registration without short name

[jambonrose]

Following discussions at DjangoCon US 2017, this PR will make it so that both full_name and short_name have blank=True, making it so that Users can (by default) register to sites using only emailand password.

The rationale is that making the field required in a custom form is easier than removing the requirement (blank=False) from the model.

This PR also modifies the test runner to check that the migration file(s) is(are) in sync with the user model.

[codecov]

Codecov Report

Merging #23 into development will not change coverage.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           development    #23   +/-   ##
==========================================
  Coverage          100%   100%           
==========================================
  Files                7      7           
  Lines              171    171           
  Branches            11     11           
==========================================
  Hits               171    171

Impacted Files	Coverage Δ
src/improved_user/migrations/0001_initial.py	100% <ø> (ø)	⬆️
src/improved_user/models.py	100% <100%> (ø)	⬆️

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