insights: display percent complete using new backfiller

[coury-clark]

Expose and display percent complete in the backfiller API

/cc @Joelkw @felixfbecker @vovakulikov

[coury-clark]

p0

[Joelkw]

@coury-clark a question here – any chance this is leaky (from a security POV)? Like could I use it to determine the existence of other repos that I can't normally see, if I see that the results for all repos I can see are already returned and yet the backfiller isn't done yet? (I don't think so because it's not possible to determine if it's just not done running historical jobs / not sure this is any leakier than the current system, which is loading in any state over current jobs. At most it would leak a rough estimate for maybe the # of repos connected to the instance total?)

[chwarwick]

p2 - Improvements to streaming progress on are the roadmap just further out.

[coury-clark]

@Joelkw

In the current state there isn't much to learn because it will always iterate over all repos, and therefore the percent progress over all repos will always be the same. In a future state with a dynamic query we might have problems, because we could imagine something like `repo:has.content(secret_string)' changing the number of repos that match and reverse engineering something from the code.

[chwarwick]

Let's allow this for now, but revisit with strat scoped insights. This should be bucketed in chunks of % to give more generic progress.