CLS2-849 change specific programme field to be a many to many field part 1

[Richard-Pentecost]

Description of change

Add new specific_programmes field to the investment project model that is a many to many field and create a migration. Add functionality such that when someone adds or changes the selected_programme for an investment project from the frontend, it updates both the current and the new field with the new value. Remove the ability to change the specific_programme field from django admin whilst this migration process is going on.

Checklist

• Has this branch been rebased on top of the current main branch?

  Explanation

  The branch should not be stale or have conflicts at the time reviews are requested.

• Is the CircleCI build passing?

General points

Other things to check

• Make sure fixtures/test_data.yaml is maintained when updating models
• Consider the admin site when making changes to models
• Use select-/prefetch-related field lists in views and search apps, and update them when fields are added
• Make sure the README is updated e.g. when adding new environment variables

See docs/CONTRIBUTING.md for more guidelines.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 96.38%. Comparing base (0f6c910) to head (1ffd2f8).
Report is 7 commits behind head on main.

Files	Patch %	Lines
datahub/investment/project/serializers.py	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5478      +/-   ##
==========================================
- Coverage   96.40%   96.38%   -0.02%     
==========================================
  Files         952      953       +1     
  Lines       22504    22516      +12     
  Branches     2021     2022       +1     
==========================================
+ Hits        21695    21703       +8     
- Misses        658      660       +2     
- Partials      151      153       +2     

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[oliverjwroberts]

PR makes first step into changing InvestmentProject.specific_programme, which is currently a foreign key, to a many-to-many field InvestmentProject.specific_programmes. Looks good to me.

Good spot with the multi-step approach (as recommended by docs/How to change the type of a field.md for those that didn't know). I'd be more than happy to assist with subsequent steps, just let me know!