Add some tracking to help us understand our users

[tomodwyer]

Using Plausible Analytics, specifically the Goals functionality, we should track:

• If a visitor is logged in
• If a visitor has admin access
• What 404 pages visitors hit
• If a visitor clicks on a video on the home page
• What visitors are searching for

[tomodwyer]

Closed in #2216

• If a visitor is logged in
• If a visitor has admin access
• What 404 pages visitors hit
• If a visitor clicks on a video on the home page

To be moved to a new issue:

• What visitors are searching for

[tomodwyer]

The initial PR was missing the custom events code, rectified in #2230

[tomodwyer]

Closed as complete, pending the search task being completed in #2231