Verify that email addresses are valid before saving request documents

[jsf9k]

🗣 Description

This pull request modifies the code to verify that email addresses are valid before saving request documents.

💭 Motivation and context

Resolves #94.

🧪 Testing

I deployed this change to database1.prod-a.cyhy, exported a customer with cyhy-export, then tinkered with the email addresses and attempted to import said customer via cyhy-import. WHen I used an valid email address the import was successful and when I used an invalid email address the import failed with an error similar to the following:

$ cyhy-import --force REDACTED.json
Traceback (most recent call last):
  File "/usr/local/bin/cyhy-import", line 186, in <module>
    main()
  File "/usr/local/bin/cyhy-import", line 177, in main
    success = import_file(db, args["FILE"], args["--force"], args["--init-stage"])
  File "/usr/local/bin/cyhy-import", line 158, in import_file
    return import_request(db, request, filename, force, init_stage)
  File "/usr/local/bin/cyhy-import", line 145, in import_request
    doc.save()
  File "/usr/local/lib/python2.7/dist-packages/cyhy/db/database.py", line 1310, in save
    raise ValueError(email_address + " is not a valid email address")
ValueError: redacted@@redacted.com is not a valid email address

✅ 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 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 new and existing tests pass.

✅ Post-merge checklist

• [x] Deploy this change to database1.prod-a.cyhy.

[jsf9k]

I'll wait for @mcdonnnj's review before merging this one.