[FEATURE] Call mixpanel from background

[tombeckenham]

Issue to be solved

We chose to send the event to mixpanel by messaging the UI from the background script. The UI then sends the track event to mixpanel.

There are two problems with that:

1. If the window is closed, then no event is sent. e.g. on_ramp_clicked never gets sent as the window is closed the second the user clicks on Moonpay
2. If the window is open multiple times, then multiple events get sent for the same thing - e.g. if the user is imporing a wallet, then opens the popup to do something

That's on top of the user being able to block it using ad block of any sort

Suggest A Solution

We can use the Ingestion APIs instead of the SDKs https://developer.mixpanel.com/reference/ingestion-api

There is also some discussion of workaround as the sdks can't be used in MV3 extensions:
mixpanel/mixpanel-js#304
mixpanel/mixpanel-js#438
https://gist.github.com/doc-han/6c998058744eebafc43c132bdc28bd00