auto convert old cons data to new cons data

[struan]

This adds a script, plus updates all the importers to automatically convert suitable datasets from the old constituency data to the upcoming ones. Essentially it does anything that is a percentage and is evenly distributed over the area or people in the area. It also adds a notice to indicate where a dataset has been automatically converted:

Fixes #360

[codecov]

Codecov Report

Attention: 58 lines in your changes are missing coverage. Please review.

Comparison is base (1030abd) 84.51% compared to head (85de704) 83.44%.

❗ Current head 85de704 differs from pull request most recent head 9602176. Consider uploading reports for the commit 9602176 to get more accurate results

Files	Patch %	Lines
hub/transformers.py	24.52%	40 Missing ⚠️
hub/models.py	71.42%	8 Missing and 2 partials ⚠️
hub/management/commands/base_importers.py	64.28%	4 Missing and 1 partial ⚠️
hub/views/area.py	89.47%	1 Missing and 1 partial ⚠️
utils/constituency_mapping.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                      Coverage Diff                      @@
##           multiple-geometry-staging     #365      +/-   ##
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- Coverage                      84.51%   83.44%   -1.08%     
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  Files                             97      100       +3     
  Lines                           2829     2942     +113     
  Branches                         285      300      +15     
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+ Hits                            2391     2455      +64     
- Misses                           369      414      +45     
- Partials                          69       73       +4     

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[zarino]

@struan I’ve attempted to tidy up the display of the "disclaimer" in the template, by moving it into the model. My instinct, though, is that the string explaining the conversion should be set by the script that actually does the conversion, rather than us having to guess which generation was converted to/from what.

In fact, perhaps DataType.auto_converted (a BooleanField) should actually be DataType.auto_converted_from_area_type (a ForeignKey to AreaType, defaulting to None) so that the view/template can automatically fetch the name and details of the AreaType from which the dataset was auto-converted?

[struan]

I've made some updates which I think answer most of your issues. I'm not totally sure about the Foreign key thing because I'm not sure what we'd do with it.

[zarino]

@struan I’ve committed a new page that explains the conversion process, and have linked to that from the disclaimers on the area page. Could you check the links all work fine, and then merge if you’re happy?