[SE] Added support for interim personal identity numbers (interimspersonnummer)

[ovidner]

Added support for interim personal identity numbers (interimspersonnummer) to the existing SEPersonalIdentityNumberField. Interim numbers are used instead of coordination numbers by Swedish educational institutions, within the Ladok system. It is disabled by default since most projects will probably not need (or want) it.

There is more information about interimspersonnummer available (in Swedish) at https://wiki.swami.se/display/Inkubator/norEduPersonNIN+och+Svenska+Personnummer and https://confluence.its.umu.se/confluence/display/LDSV/Interimspersonnummer

All Changes

• Add an entry to the docs/changelog.rst describing the change.

• Add an entry for your name in the docs/authors.rst file if it's not
  already there.

• Adjust your imports to a standard form by running this command:

  `isort --recursive --line-width 120 localflavor tests`
  

[codecov-io]

Codecov Report

Merging #308 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #308      +/-   ##
==========================================
+ Coverage   96.28%   96.29%   +<.01%     
==========================================
  Files         152      152              
  Lines        4230     4239       +9     
  Branches      579      582       +3     
==========================================
+ Hits         4073     4082       +9     
  Misses         98       98              
  Partials       59       59

Impacted Files	Coverage Δ
localflavor/se/utils.py	88.09% <100%> (+0.59%)	⬆️
localflavor/se/forms.py	100% <100%> (ø)	⬆️

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[benkonrath]

PR looks good. Thanks for this!