Integration

[Big-Lolo]

This pull request adds the ability to send reminder emails to accepted hackers for an event and improves the flexibility of sending Slack invitation emails. The changes introduce a new endpoint for sending reminders, update the Slack invitation flow to accept a custom URL, and add a new internal mail template for reminders.

New reminder email functionality:

• Added a new POST endpoint /{event_id}/send_reminder_mails/ in router_v1.py to send reminder emails to all accepted hackers of an event. This endpoint ensures only authorized users can trigger reminders and handles errors gracefully.
• Implemented send_reminder_mails method in Event/service.py that sends reminder emails to accepted hackers who have not confirmed attendance, generating a confirmation token if needed and including event and user details in the email.

Improvements to Slack invitation email flow:

• Changed the Slack invitation endpoint from GET to POST and now require a slackUrl parameter, allowing dynamic Slack invite links to be sent.
• Updated the send_slack_mail method in Event/service.py to use the provided slackUrl for each email and to send the created email immediately. [1] [2]

Mail template updates:

• Added a new internal mail template identifier EVENT_HACKER_REMINDER in internall_templates.py for sending reminder emails.