Incorporate submitter activity

Graph is currently based completely on commenter activities. Should be trivial to determine active submitters and incorporate that information into subreddit activity and mutual user calculations (assuming this data is available on GBQ, which I believe it is). There are probably many active subreddits that have a low comment volume but high submission volume.

If possible, I'd also like to incorporate the subreddit mentions graph, but I'm not sure what the analogous version of the mutual activity ratio would be. Maybe I could come up with some kind of clever rescaling between a nodes observed in-degree/strength in the mutual commenter graph and the nodes degree/strength in the mentions graph. Incorporating mentions does mean I'd have to deal with abbreviations, although this shouldn't be a huge issue.

No files, tests, or entry points are named. Start by locating the code that builds the commenter-activity graph and mutual user calculations, then verify whether submitter activity is available in GBQ; done would require incorporating submissions and defining a defensible approach for the optional subreddit-mentions graph.

Written by the indexing model from the issue text.