feat(metrics): set metrics flow data via data attrs

[chenba]

Because:

• we need to able to get metrics flow data directly in React/Settings without relying on query params

This commit:

• injects a flow id and begin time into the HTML of the React page
• sets the metrics flow data for the app in the precedence of 0) direct argument, 1) query params, 2) body data attrs

[chenba]

@chenba In my local testing the flowId from Backbone app (email first) -> React app (signup) is not maintained. I wonder if it makes sense to make the flowId in query params the source of truth since we currently set that when coming from content-server.

The flow id and begin time in the query params should have higher precedence. That's by default but I also pass the flowQueryParam into the init to make that explicit. If you look at the requests, which flow id do you see in the payload?

[chenba]

Is it alot of work to set this value from the url query params in the body element?

Probably not? I have not considered it. What's the advantage of doing so?