Add a success dialog when a user completes registration form

🗣 Description

Upon submitting a request for an account, the User hits the register button. This then triggers the approval process and an email is sent to the User to let them know their account is pending approval. From the UX perspective a message should be displayed in the UI also letting the User know their account is pending approval

💭 Motivation and context

To give customers a way to see that filling out the registration form was a success.

🧪 Testing

Upon "Register" the following message is relayed to the user; "Thank you for requesting a Crossfeed account, you will receive notification once this request is approved."

📷 Screenshots (if appropriate)

✅ Pre-approval checklist

[x] This PR has an informative and human-readable title.
[x] Changes are limited to a single goal - eschew scope creep!
[x] All future TODOs are captured in issues, which are referenced in code comments.
[x] All relevant type-of-change labels have been added.
[x] I have read the CONTRIBUTING document.
[x] These code changes follow cisagov code standards.
[x] All relevant repo and/or project documentation has been updated to reflect the changes in this PR.
[x] Tests have been added and/or modified to cover the changes in this PR.
[ ] All new and existing tests pass.

✅ Pre-merge checklist

[ ] Revert dependencies to default branches.
[ ] Finalize version.

✅ Post-merge checklist

[ ] Create a release.

cisagov / crossfeed