feat(metrics): Add the `isMigration` field to the reported metrics.

Only reported for FxDesktop. Sent when the browser is migrating a Sync user.

See https://bugzilla.mozilla.org/show_bug.cgi?id=1100666

fixes #1906
feat(metrics): Add the `campaign` metric.

campaign is used by the Desktop Fx crew to indicate which campaign they are running.

fixes #1792
Change `isMigration` to `migration`, expect a string.
Merge pull request #1928 from mozilla/issue-1906-isMigration

feat(metrics): Add the `migration` and `campaign` fields to the reported metrics.

I see two thumbs up, one from @kparlante, the other from @zaach. double r+!