Email changes

[tymees]

This PR addresses #529. It was a pain, but I think I got everything right.

As a bonus, I also implemented #623, as it was kinda related.

I cannot wait till we have the new mailing code. Keeping multiple versions in sync is a pain

[tymees]

So this var isn't used in the template, as Desiree's text didn't contain it. It's here because it was in the other reminder mail code I copied, so I left it in for the inevitable 'why isn't there a link' comments.

Feel free to nudge me to add it to the e-mail, but I'm tired atm :P

[EdoStorm96]

I could not find any faults. Respect for doing this haha. 😅 I suppose it would indeed be best to add a link to reminder_supervisor already, if you can find the energy.

How do cron jobs work though? Where do these reside for ethics?

[tymees]

How do cron jobs work though? Where do these reside for ethics?

Cron jobs are set on the server OS; Under the hood, it's a program that runs in the background, that just fires off 'shell commands' depending on the configured interval. We configure it to run a django management command ('send-reminders').

As described in #315, we run this:
0 7 * * * /<snip>/data/env/bin/python /<snip>/data/source/manage.py send_reminders >/dev/null 2>&1

This means 'on minute 0, hour 7, any day, any month and any year, execute the following command'.

(So, 15 7 1 3 2025 would be 'run this command on the first of march, 2025 at 7:15'. Nobody would do this, as this would only run once ever)