Squash migrations using the "nuclear" option

[Andrew-Dickinson]

Removed all existing migration code, and ran python src/manage.py makemigrations to create new ones from scratch

Copied over the manually written migration for the readonly database role

Closes #594

Procedure to roll out this change:

Everywhere that we have a DB, drop all the tables (super destructive option, make sure you have a backup first):

DROP SCHEMA public CASCADE;
CREATE SCHEMA public;

And run the new migrations

python src/manage.py migrate

Then to get the data back, import from a backup, run the import script, use scripts/import_spreadsheet_dump.sh, etc.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.21%. Comparing base (fc2c805) to head (e63f3e5). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #634 +/- ## ======================================= Coverage 94.21% 94.21% ======================================= Files 80 80 Lines 3284 3284 ======================================= Hits 3094 3094 Misses 190 190 ```

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