VaNotify - Add confirmation email to 1990

[nathanbwright]

Summary

• Adds confirmation email to form 1990
• Sending email is behind a feature flag which will be removed once once we have incrementally rolled this out and verified that it is working as expected.

Related issue(s)

• https://app.zenhub.com/workspaces/forms-strike-team-620c2914e703390013c4b414/issues/gh/department-of-veterans-affairs/vanotify-team/765

Testing done

• Rspec
• Will test manually in staging before releasing to prod

Screenshots

Note: Optional

What areas of the site does it impact?

Kicks off an sidekiq job to send an email confirmation to veterans when they have successfully submitted a 5490 form.

Acceptance criteria

• I fixed|updated|added unit tests and integration tests for each feature (if applicable).
• No error nor warning in the console.
• Events are being sent to the appropriate logging solution
• Documentation has been updated (link to documentation)
• No sensitive information (i.e. PII/credentials/internal URLs/etc.) is captured in logging, hardcoded, or specs
• Feature/bug has a monitor built into Datadog or Grafana (if applicable)
• If app impacted requires authentication, did you login to a local build and verify all authenticated routes work as expected
• I added a screenshot of the developed feature

Requested Feedback

(OPTIONAL)What should the reviewers know in addition to the above. Is there anything specific you wish the reviewer to assist with. Do you have any concerns with this PR, why?