[Bexley] Adapt postcode DB script to take URL for fetching CSV.

[neprune]

[skip changelog]
closes https://github.com/mysociety/societyworks/issues/4463

Matthew:

• I rearranged the script to put flow top / business logic middle / setup data last.
• For use in production, it creates a new full file in a tempfile then moves into place. For updates it updates in place.
• It works out the type of file (full or update) from its contents.
• As we don't know what rows a COU file will contain, I thought it easiest to store the BLPU rows as well (then they can be updated if changes come in) and change the queries to check property shell then, rather than try and work out dependencies between the types.
• As we don't use full text search, I changed the tables to be normal ones with a primary key, making updating much easier. If FTS is needed in future, that can be joined on (but you can't primary key a FTS table)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.44%. Comparing base (ed34041) to head (af8c0f1).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5365   +/-   ##
=======================================
  Coverage   82.43%   82.44%           
=======================================
  Files         416      416           
  Lines       32979    32979           
  Branches     5308     5308           
=======================================
+ Hits        27187    27188    +1     
+ Misses       4219     4218    -1     
  Partials     1573     1573           

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