Send emails for new manuscripts for each subscription

[uxairibrar]

Closes #71

• Set up subscription-based email sending, fetching users with active subscriptions and sending manuscript updates.
• Created unsubscribe functionality allowing users to opt-out from specific or all subscriptions via email links.
• Formatted emails in HTML with bullet-point manuscript lists for better readability.
• Implemented monthly email scheduling using Django-Q to send emails to subscribed users about new manuscripts.
• Added an Admin action to manually trigger subscription-based emails from the admin panel.

[nuest]

Looks very much going in the right direction - I suggest to revisit this after we merge the simpler email sending PR #89

[nuest]

Not sure you already wanted input here, but I have a few minor comments.

Happy to merge soon, AFAICT only some tests are missing.

[nuest]

Looks all good, I'm only missing the tests.

[uxairibrar]

I have updated the tests

[nuest]

@uxairibrar Tests are failing. Please fix the unit tests.