Feature/custom confirmation

[otytlandsvik]

✨ Add support for custom emails to event participants

Modified /event/{eid}/confirm endpoint

• Now supports custom email body from payload
• Added /event/{eid}/confirm-message endpoint to fetch default confirm mail with event data

Added /event/{eid}/mail endpoint

Allows admins to send custom emails to event participants. Can choose between

• All participants
• All confirmed participants

🎨 Formatting

Ran autopep8 on various files

⚠️ Should be merged simultaneously with corresponding frontend PR

[FredrikMorstad]

Looks good and a very nice feature to have 🙌 Something to consider is to run the email sending in the background as it takes some time to send all the mails