insights: initial live preview for creation of insights over all repos

Created by: chwarwick

Currently the live preview of an insight's data series is only available when creating an insight over a named set of repositories. We want to enable a preview experience when creating an insight over all repositories that provides a subset of data that enables the user to feel confident that the entered search query is correct.

Running the preview over a small subset of all repositories can meet this goal if and only if the selected repositories contain data relevant to the query and the user is aware which repositories have been used during creation of the preview.

In the current system state the user creating the insight has the best knowledge to decide which repositories have the relevant data.

When creating an insight over all repositories provide an input for the user to specify one or more repositories to use for the purpose of a live preview.