[Armadillan]

Fixed broken HTML in email templates.

Private bookings now send an ICS calendar invite to BilF so I don't need to add them to my calendar manually.

Also includes new tests for the calendar feature and car-related emails generally.

ChatGPT wrote 99% of this.

[georgelgeback]

@Armadillan Hi! I changed some things since I found the time to do so, if you don't like anything or think it doesn't mesh with how BilF works just tell me and Ill revert it:

• Added special email that gets sent out when someone deletes a booking or converts it to a council booking
• When a private booking gets deleted it should now delete it from your calendar too (or maybe you'll get a one-click unbook, not sure)
• You might get emails every time a private booking gets confirmed, which might be very useful if someone else does it or very spammy if you mostly do it
• The name of the person booking isn't on the attendee list of the calendar anymore as a simple fix to a bug. It's in the description still so it will hopefully be fine. I can add it to the title if you need it close at hand or try to get it back on the attendee list.

Everything is LGTM. If you're fine with my changes just thumbs up this comment and I'll merge when I got time for it.