Send email notifications; adds more audit and error logging

[simon-20]

This PR:

• Refactors add_user_to_org to use assert_precondition_met, and begins to unify the format of the audit and app logs
• Stops users from being able to attempt to join the same organisation twice (resolving Add validation to POST /{user}/reporting-org to stop users adding themselves to an organisation twice #60)
• Adds jinja for email templating, pytest-watcher for easier developing, Azure Email Communication Service library, and email validation libraries
• Adds an EmailGenerator class for generating emails from templates
• Adds a background worker processor, and email sending function
• Modifies create_reporting_org to notify Support upon reporting org creation (resolving Send support@iatistandard.org an email when a new organisation is created in IATI Account #50)
• Modifies add_user_to_org to notify org admin when someone joins an org (resolving Send the reporting org administrator an email when someone requests to join the organisation #49)

Note: this PR adds several new environment variables, so the salt deployment infrastructure will need updating before this is deployed.

[chrisarridge]

This looks good @simon-20 . I think my three comments are:

1. Should we change "IATI Registry" to something else?
2. I was a bit confused between the org_human_readable_name and org_name in the templates, from what I could tell they are sort of synonymous in that org_name is set to the human readable name?
3. There is one place where, when the application log is set to debug level, email details and PII will appear unencrypted in plaintext in the application log. I think we should move that to the audit log.

I'm not sure these are blockers, hence approved.

[chrisarridge]

All good @simon-20