replace abandoned mail library

[Jumpy-Squirrel]

go-gomail/gomail#182

Our mail library is abandonware, and it currently only supports attachments from files, but we want to store base64 encoded attachments in the database / expose via API.

One possible alternative might be https://github.com/xhit/go-simple-mail

We will have to ensure to maintain backwards compatibility, maybe temporarily import both libraries.

Update:

There's now a maintained fork for our original library that also supports attachments from writers in-memory. Should cause fewer code changes. https://github.com/Shopify/gomail

We should try that first.