Enterprise offer usage email updates

[iloveagent57]

• Install edx-braze-client and create a new EdxBrazeClient class with the library client class as its base.
• Use the new client for sending API-triggered campaign messages for the Enterprise Offer Usage email.
• The send_offer_usage_email_via_braze task now takes a dict of {email: lms_user_id} instead of a simple list of email addresses as its first parameter.
• Attempt to create_recipient with the lms user ID as the external ID for each email to which this message is sent.

Used by: #169

Merge checklist:

• Any new requirements are in the right place (do not manually modify the requirements/*.txt files)
  • base.in if needed in production
  • test.in for test requirements
  • make upgrade && make requirements have been run to regenerate requirements
• Version bumped

Post merge:

• Tag pushed and a new version released
  • Note: Assets will be added automatically. You just need to provide a tag (should match your version number) and title and description.
• After versioned build finishes in GitHub CI, verify version has been pushed to PyPI
  • Each step in the release build has a condition flag that checks if the rest of the steps are done and if so will deploy to PyPi.
    (so basically once your build finishes, after maybe a minute you should see the new version in PyPi automatically (on refresh))
• PR created in ecommerce to upgrade dependencies (including ecommerce-worker)
  • This must be done after the version is visible in PyPi as make upgrade in ecommerce will look for the latest version in PyPi.
  • Note: the ecommerce-worker constraint in ecommerce must also be bumped to the latest version in PyPi.
• Deploy ecommerce
• Deploy ecomworker on GoCD.
  • While some functions in ecommerce-worker are run via ecommerce, others are handled by a standalone AMI and must be
    released via GoCD.