SGM-6993 - Custom event instrumentation for PostHog

[gitstart-app]

This PR was created by GitStart to address the requirements from this ticket: SGM-6993.
This ticket was imported from: SGM-6993

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Description:

Sets up custom event capture for the website using PostHog.

Events captured:

• “Login” button clicks in nav bar

  (event_name = ‘login_click’. Properties : current_page = The URL the user is viewing when they click the login button.)

• Cody CTA button clicks (on the navbar)

• Cody CTA button clicks (on a page)

• Authentication button clicks

• “Contact sales” button slicks

• Form submission

• [Sourcegraph Issue] Custom event instrumentation for PostHog #6993

• [Ticket] https://clients.gitstart.com/sourcegraph/3436/tickets/SGM-6993

Each custom event requires an event name, and also in some cases custom properties for each event were captured. Check detailed documentation here: https://docs.google.com/document/d/1Lw3cJw6ZKsKoJff9ahYtX_Dg_ucZFnMDzgGc9qgBMg0/edit

DEMO:

https://www.loom.com/share/0040c2caa4a8453881c76f47ad37186e?sid=d358076a-5f25-41c4-94cb-890cfdcf6ad2

Test cases:

• All documented events are captured and displayed in the posthog account activity logs

• Each click is tracked and captured as requested in the documentation

• Eventnames match what was requested in the documentation

• Current page / position is returned in the captured event on the posthog activity section

Test plan:

-Login to the posthog account at https://us.posthog.com/

-Request login credentials from PM or TM

-Donot submit the form on "contact/request-info"

-Check demo video for testing scenarios

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[iskyOS]

Tested functionality and data to PH, lgtm on that side