database: parameterise indexable repos min stars
Created by: tsenart
This commit introduces an environment variable
SRC_INDEXABLE_REPOS_MIN_STARS
that can be used to change the minimum
number of stars neeeded for a public repo to be indexed by Zoekt on
sourcegraph.com.
Additionally, we change the threshold from 13 to 8, in order to grow the index to ~1.5M repos.