[bug 1107083] Backfill versions

This has a data migration that backfills the version value where the
following are all true:

1. version == ''
2. product and user_agent are non-empty strings
3. product == inferred product from user_agent

If all those are true, then we copy browser_version to version.
Merge pull request #404 from willkg/1107083-version-backfill

[bug 1107083] Backfill versions