FFS-2162: Set up mixpanel and use it to track page views

[tdooner]

Ticket

Resolves FFS-2162.

Changes

• Add MixpanelEventTracker proof of concept

Context for reviewers

This is just an initial implementation. Plans for this PR:

• Add environment variable to terraform

Plans for a subsequent PR that finishes the NewRelic -> Mixpanel transition:

• Call mixpanel.identify at some point to associate the distinct_id with the user's device id
• Use maybe_later gem to move event tracking to happen after the body is sent to the client (for performance)
• Audit event names to try and ensure consistency of "[subject][action][object]", e.g. "CaseworkerLoggedIn" or "ApplicantDownloadedIncomePDF" where possible.
• Try and simplify the metric tracking somehow, possibly by moving the tracking methods onto a separate module

Setup instructions:

1. Engineers will need to get the MIXPANEL_TOKEN value from the shared 1password .env.development.local file.
2. Engineers will need to create Mixpanel accounts.

Acceptance testing

• No acceptance testing needed
  • This change will not affect the user experience (bugfix, dependency updates, etc.)
• Acceptance testing prior to merge
  • This change can be verified visually via screenshots attached below or by sending a link to a local development environment to the acceptance tester
  • Acceptance testing should be done by design for visual changes, product for behavior/logic changes, or both for changes that impact both.
• Acceptance testing after merge
  • This change is hard to test locally, so we'll test it in the demo environment (deployed automatically after merge.)
  • Make sure to notify the team once this PR is merged so we don't inadvertently deploy the unaccepted change to production. (e.g. :alert: Deploy block! @ffs-eng I just merged PR [#123] and will be doing acceptance testing in demo - please don't deploy until I'm finished!)

[GeorgeCodes19]

Nice - I'm not sure why we didn't implement something like this for the NR event tracker.