Mailjet email provider and email meeting confirmation

@DSASanFrancisco/portal-members

This is a PR that:

• adds Mailjet as an email provider
• generates one-time use email links for people to confirm their meeting attendance

See membership_ui!363 (merged) for more details

I have:

• Included a summary of the changes
• Linked all relevant Trello tickets to this request
  • https://trello.com/c/65pdUulZ/280-sv-email-verification-of-member-sign-ups
• Linked all relevant GitLab issues to this request (optional -- prefer Trello, but GitLab is easier for fast-follow cleanup issues)
  • svdsa/home#8
• Listed all dependent merge requests in this summary (optional)
  • !306 (merged)