Analytics/metrics setup

[microbit-matt-hillsdon]

Description

Add a structured approach to metrics. Consider learnings from v2.

Todo

• Sentry
• GA
• Cookie notice - this is covered by the versioner.
• Integrate it for key editor actions. Detail to be addressed separately, including the extent to which we aim for comparability with v2.

I've not added our legacy metrics endpoint. We can discuss/add that in future.

Conclusion

Sentry and GA are working on our staging deployment.
Notes about first non-staging release added to #28.
We'll evolve the interface as we look at individual actions and how best to log them.

[microbit-matt-hillsdon]

GA confirmed working (using our demo account).

[microbit-matt-hillsdon]

Triggered a test Sentry error and that's working fine too.