Distinguish email types by Sendgrid email category

[ymilki]

Sendgrid has a feature to categorize emails: https://docs.sendgrid.com/for-developers/sending-email/categories

While debugging emails in Sendgrid, we thought it would be useful to distinguish between the different emails that we send.

In particular - the meeting request email and the successful/failure match emails.

[ymilki]

The meeting request email would be the first spike. And in this case, there were no match emails.

With categories, we could see when certain types of email weren't sent out.