Add mailing and RSVP functionality for Events

[HarshRaj2717]

(This is a leftover from #8) Event handling feature requires a way to send emails to some selected users (or all users) which are registered for that event, these mails can be for promotions, RSVP, event guidelines, etc.

• [x] Admins should be able to send emails to some or all registered users of that event from admin panel (prefer markdown or HTML format for mail body)
• [ ] ~Admins should be able to send RSVP mails (with a link for RSVP) to selected users of that event~
• [ ] ~Users should be able to RSVP for the event after registering~
• [ ] ~Admins should be able to get a list of RSVP'ed users and send RSVP confirmation or events tickets to them~

[DevilsAutumn]

Lets pick RSVP functionality later on.