Add an admin flow for the mapit_import command.

[neprune]

• Extracts out core area adding logic in mapit_import.py into a 'pure' function.
• Sets up a custom admin view that runs this function synchronously.

Note that the area adding logic has been kept 'as is' and many paths still don't have coverage yet.

Link to new view in admin page:

Form submission:

Successful run:

Unsuccessful run:

[codecov[bot]]

Codecov Report

Merging #397 (92bbbd9) into master (a784963) will increase coverage by 2.67%. The diff coverage is 78.68%.

@@            Coverage Diff             @@
##           master     #397      +/-   ##
==========================================
+ Coverage   72.43%   75.10%   +2.67%     
==========================================
  Files          40       45       +5     
  Lines        2380     2740     +360     
==========================================
+ Hits         1724     2058     +334     
- Misses        656      682      +26     

Impacted Files	Coverage Δ
mapit/admin.py	97.36% <ø> (ø)
mapit/management/command_utils.py	69.30% <50.00%> (-1.70%)	:arrow_down:
mapit/add_areas_from_file/core.py	56.58% <56.58%> (ø)
mapit/management/commands/mapit_import.py	74.25% <71.83%> (+21.80%)	:arrow_up:
mapit/tests/add_areas_from_file/skeletons.py	92.23% <92.23%> (ø)
mapit/tests/add_areas_from_file/test_admin_flow.py	98.63% <98.63%> (ø)
mapit/add_areas_from_file/admin_flow.py	100.00% <100.00%> (ø)
mapit/tests/add_areas_from_file/test_cli.py	100.00% <100.00%> (ø)
mapit/tests/add_areas_from_file/test_core.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[neprune]

@davea sharing early in case of any feedback on top-level approach - not blocked.

[neprune]

@davea Ready for review. Very sorry for the big PR, I couldn't find a good way to break it down.