TET-880: Use secure cookie flag everywhere

[baarkerlounger]

Description of change

https://uktrade.atlassian.net/browse/TET-880

The Django admin messages contrib module takes it's cookie settings from the general Django config. The SESSION_COOKIE_SECURE setting defaults to off, which means the messages cookies were also being created with the flag off, for example in the request below. This was flagged in the most recent pen test. e.g.:

POST http://localhost:8000/admin/company/advisor/f4a33f93-6303-4a7e-a813-ab4c204df2e3/change/

Before:

After:

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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.64%. Comparing base (a2b9325) to head (31ac881).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5956      +/-   ##
==========================================
- Coverage   96.65%   96.64%   -0.01%     
==========================================
  Files        1077     1077              
  Lines       25330    25330              
  Branches     1669     1669              
==========================================
- Hits        24482    24481       -1     
- Misses        691      692       +1     
  Partials      157      157              

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[dredmonds]

LGTM👍

[stuart-mindt]

LGTM