[59] Add Mixpanel analytics

[braxex]

Summary

Mixpanel is a customer and product analytics platform that we use to track analytics related to traffic and user behavior. This helps us better meet user needs by monitoring application usage so we can improve product performance and behavior.

It's best practice to separate your tracked data by environment. We have two separate Mixpanel projects set up: Test (used for testing conducted in lower environments) and Prod (used for tracking production data). This data can be seen after invitation and authentication. For current tech challenge team members, you can authenticate with Google SSO.

Closes #59 (issue)

Type of change

• [x] New feature (non-breaking change which adds functionality)
• [x] This is a documentation update

How To Test

1. Log in to Mixpanel test project (see above). Click 'Events'.
2. Click on 'Apply to this school' button on main page.
3. View your events in Mixpanel (note: adblockers may block this -- best to try in incognito).

Checklist:

• [x] My code follows the style guidelines of this project and has no linting errors
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation, including any applicable ADRs
• [x] My changes generate no new warnings
• [x] I have added tests that fail without these changes
• [x] New and existing tests (unit, integration, accessibility) pass locally
• [x] Documentation updated
• ~[] If there are security concerns they are addressed or ticketed after being discussed~

[codeclimate[bot]]

Code Climate has analyzed commit 02c4febd and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (90% is the threshold).

This pull request will bring the total coverage in the repository to 100.0% (0.0% change).

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