Improve notification emails

[AlbertoPdRF]

SendGrid should be configured so emails sent through it get signed as coming from @speedcubingmadrid.org, as that should avoid our emails getting tagged as spam, specially by Microsoft.

Also, we can only send 100 emails/day for free, so when two competitions get announced on the same day many emails don't get sent, so we should figure out how to handle this or if we should pay to be able to send more emails.

[AlbertoPdRF]

The second part of the issue can easily be fixed after d5b2022 -- we now have a G Suite group containing all active subscribers who want to receive notifications, so emailing the new competition notification just there should make the trick!