repo discovery using pagerank

[JamesKunstle]

uses pagerank-centrality to identify users who are the most central in the 'ecosystem' seed we're considering, and uses the events of those users to find the repos that are the most mutually-relevant as a re-seed.

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[christinaexyou]

@JamesKunstle mostly nits, overall fantastic work and I loved the explanation of Next Steps at the end. Can't wait to start putting the pieces of the puzzle together :)

[JamesKunstle]

@christinaexyou just updated everything!

[christinaexyou]

@JamesKunstle lgtm ! I don't have write access so I can't approve but I think it's ready to merge.

[JamesKunstle]

@hemajv I've responded to your comments! Thanks so much for the review

[oindrillac]

Thanks @JamesKunstle LGTM