Implement SMTP email for better reliability

[fabd]

Implement SMTP email sending with Google SMTP (instead of using php mail() on the shared hosting).

Ideally, we will be using XOauth2.

Address the following:

• improve email delivery, with less emails going to Spam folders
• remove php mail() dependency, to simplify the process of moving to eg. LINODE

[fabd]

We're using free Google SMTP with 2FA but that will do for now - setting up XOAuth is a major PITA.

Still much better than mail() on the shared hosting.

[fabd]

Closed by #327