Refactor [github] initialization (#1861)

This creates a new convenience class which consolidates all the Github initialization. It supports dependency injection and facilitates refactoring the persistence along the lines of #1205.

Also ref #1848

lib/github-auth.js

@@ -1,6 +1,5 @@
 'use strict'
 
-const path = require('path')
 const crypto = require('crypto')
 const log = require('./log')
 const queryString = require('query-string')
@@ -14,12 +13,7 @@ const mapKeys = require('lodash.mapkeys')
 // with a JsonSave object.
 let githubUserTokens = { data: [] }
 
-function scheduleAutosaving(config) {
-  const { dir: persistenceDir } = config
-  const githubUserTokensFile = path.resolve(
-    persistenceDir,
-    'github-user-tokens.json'
-  )
+function scheduleAutosaving(githubUserTokensFile) {
   autosave(githubUserTokensFile, { data: [] })
     .then(save => {
       githubUserTokens = save