Rewrite tree search queries using MongoDB $graphLookup

[amoeba]

When I originally wrote this codebase, MongoDB didn't have a built-in way to do graph traversal so my solution was to use a loop (originally, recursion), see https://github.com/amoeba/treestats.net/blob/main/helpers/chain_helper.rb#L68. The problem with this is that it's slow and scales poorly: We issue a query for each node in the tree which hammers the database and can take many seconds to finish.

Since MongoDB 3.4, the $graphLookup aggregation stage seems like it can do this all for us in one query which should be orders of magnitude faster.