var -> let
var -> let

The entire codebase was using "let" instead of "var" except for the worker,
which previously wasn't being run through babel, but is now. So I've updated it
to use "let" like the rest of the codebase.

dattnetwork/datt,yoursnetwork/yours-core,dattnetwork/datt,ryanxcharles/datt,ryanxcharles/datt,yoursnetwork/yours-core