Re-index all repos after `search.largeFiles` changes

Created by: nicksnyder

A customer added the search.largeFiles configuration and asked how long it will take to re-index all repositories.

https://sourcegraph.slack.com/archives/G9EN3TJDD/p1563208733221700

My assumption is that we don't automatically reindex a repository unless contents have changed, but this isn't a great outcome because it means that this configuration change is not guaranteed to have an effect in any bounded amount of time.

1. Is there a manual way that customers can force all repositories to be indexed?
2. Can we make this happen automatically when search.largeFiles changes?