Fix #54 - Fetch remote accounts by URL from mentions

Fetching atom extracted from FetchRemoteAccountService and FetchRemoteStatusService
into FetchAtomService. Mentions of the constant "http://activityschema.org/collection/public"
skipped as it's not a real URL/user.

spec/services/fetch_remote_account_service_spec.rb

@@ -0,0 +1,4 @@
+require 'rails_helper'
+
+RSpec.describe FetchRemoteAccountService do
+end