insights: create search insight changing from scoped to all repos modifies time granularity unit

Created by: chwarwick

When creating a search based insight checking the Run your insight over all your repositories changes the granularity units to months.

Steps to reproduce:

1. Click Create a new Insight and select Track Changes
2. Enter a single repository, series name and search query
3. Change the Granularity to 1 Week
4. The Live Preview Appears at 1 week intervals
5. Check the Run your insight over all your repositories

Expected behavior: The Granularity should remain at 1 Week

Actual behavior: That Granularity changed to 1 Month