TP2000-1067 Fix measure editing footnote save

[dalecannon]

TP2000-1067 Fix measure editing footnote save

Why

Editing a measure to add a new footnote requires pressing "Add another footnote" before "Save" otherwise the footnote is not added to the measure.

What

• Checks submitted form data for footnotes other than those already found in session data (i.e those that have been added via "Add another footnote" button)
• Adds test
• Moves formset prefixes to constants file
• Tidy up footnote formset template

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (031a5da) 92.75% compared to head (b01a0b5) 92.77%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1050      +/-   ##
==========================================
+ Coverage   92.75%   92.77%   +0.02%     
==========================================
  Files         460      460              
  Lines       34062    34120      +58     
  Branches     2596     2603       +7     
==========================================
+ Hits        31593    31656      +63     
+ Misses       1971     1968       -3     
+ Partials      498      496       -2     

Files	Coverage Δ
commodities/tests/test_views.py	100.00% <100.00%> (ø)
common/views.py	80.41% <100.00%> (ø)
geo_areas/tests/test_views.py	99.24% <100.00%> (ø)
measures/constants.py	100.00% <100.00%> (ø)
measures/forms.py	96.11% <100.00%> (+0.35%)	⬆️
measures/tests/conftest.py	91.40% <100.00%> (ø)
measures/tests/test_forms.py	99.62% <100.00%> (+<0.01%)	⬆️
measures/tests/test_views.py	100.00% <100.00%> (ø)
measures/views.py	95.48% <100.00%> (+0.19%)	⬆️
quotas/tests/test_views.py	100.00% <100.00%> (ø)
... and 1 more

... and 2 files with indirect coverage changes

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