admin-analytics: fix to count registered users only once across multiple browsers/IDEs

Created by: erzhtor

Currently, "Analytics / {Feature}" > "Unique users" counts registered users multiple times if they were using on multiple browsers or IDEs. This is true for the "analytics / users" > "Active Users" metric.

This PR fixes to count registered users only once.

Note:

• that anonymous users will still be counted multiple times since there is no way to distinguish between devices.
• that "forever" deleted (aka nuked) users' devices/browsers/IDEs will still be counted multiple times since we no longer keep their original user ids, thus falling back to anonymous_user_id, which is unique per device/browser/IDE.

Test plan

• check the diff
• sg start
• Open http://localhost:3080/site-admin/analytics/search and the rest of the analytics feature pages
• Check that "total/unique" charts load without any errors