Re-add SMTP email provider

[sea-kelp]

Description of Changes

The current EmailClient class only supports sending emails from a GSuite email account.

This change allows OpenOversight to auto-detect which email implementation to use (Google, SMTP, or Simulated) based on which of the following is configured:

• Google: service_account_key.json file exists and is non-empty
• SMTP: MAIL_SERVER and MAIL_PORT are set
• Simulated: No other imeplementation is configured

Notes for Deployment

Screenshots (if appropriate)

Tests and linting

• [x] This branch is up-to-date with the develop branch.
• [x] pytest passes on my local development environment.
• [x] pre-commit passes on my local development environment.

[sea-kelp]

@michplunkett - I just tested these changes using SMTP. Would you mind testing with a Google Workspace account?

[michplunkett]

Some small nits here and there, but it otherwise looks great! I'll do some testing on it Monday afternoon and stamp this bb with an 'Approved' afterward!

[michplunkett]

Tested with GSuite and made the appropriate changes. Provided you're happy with it, @sea-kelp, it's good to merge!

[sea-kelp]

@michplunkett Thanks for the changes! Please do merge this PR when you get a chance