invites to blog.crossplane.io are broken

It appears impossible currently to invite someone to be an author on blog.crossplane.io.

> Error sending email! Error sending email: Failed to send email. Please check your email settings and resend the invitation.

I debugged for quite awhile when this first happened a few months ago, but was unable to find a permanent fix at that time. The Ghost email settings show that we're using the `direct` mail option, so I suspect that we're not sending authenticated mail and email providers like gmail reject it.

The current known workaround is to follow the procedure outlined in https://blog.tiga.tech/posts/invite-users-to-a-ghost-blog-without-sending-an-email/

special notes when trying the workaround:
* get mysql connection info from `/var/www/ghost/config.production.json`
* remove the trailing `=` or `==` chars from the token when crafting the URL

workaround commands:
```
# access droplet console in https://cloud.digitalocean.com/droplets

mysql -u ghost -p
# enter password from mysql connection info

use ghost_production;
UPDATE invites SET status='sent';
SELECT email,token FROM invites;

# go to URL with https://blog.crossplane.io/ghost/#/signup/