Implement retention graph

[philbooth]

Part of mozilla/fxa#45.

This work will actually be done in mozilla/accounts-dashboard, but I'm raising the issue here to give it visibility in our planning process.

When #86 is in place, we can implement the new retention graph plotting account activity at days 0, 1, 3, 7, 14, 30, 60 and 90.

[vladikoff]

@philbooth hey, let's chat about this in the next few days on Vidyo! 📞

[philbooth]

Sounds good @vladikoff. We can liaise in IRC but fwiw today I'm free all afternoon and evening (BST), tomorrow and Friday I'm free in the afternoon but not the evening.

[vladikoff]

Draft 1

this the plan we had initially, but have to remove Cassandra from it:

Draft 2:

[ckarlof]

I'm happy to see progress on this!

[vladikoff]

Blocked by https://bugzilla.mozilla.org/show_bug.cgi?id=1207015#c20

[vladikoff]

the basic account.signed event graph is complete and linked here: https://mana.mozilla.org/wiki/display/CLOUDSERVICES/Retention+Metrics

We can now proceed making other retention graphs