Closes #453

With the release of Django 5.0, Python 3.9 is no longer supported. Python 3.9 is also incredibly old at this point, so updating to a more recent version can be beneficial for the extra features and fixes.

This PR upgrades dependencies to support Django 5 and Python 3.12.

The most notable changes are:

• All GitHub CI, Docker containers, and Heroku files have been updated to use Python 3.12 instead of 3.9.
• Django 4.2 deprecated the DEFAULT_FILE_STORAGE and STATICFILES_STORAGE options, adding a new STORAGES option instead. The existing settings have been updated to use this new setting.
• All existing dependencies have been updated to the most recent versions listed on PyPI.
• Django 4.1 deprecated logging out with a GET request, and the ability to do so is removed in Django 5.0; this means that several places need to be adjusted to use POST requests instead. In particular:
  • The logout button now calls a utility method to send a POST request (following redirects)
  • A new Cypress command for logging out a user has been added, and existing Cypress tests that have a step logging users out were also updated. (New tests have been added to ensure that no regressions occur in the future.)