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.

app/services/fetch_remote_account_service.rb

@@ -0,0 +1,22 @@
+class FetchRemoteAccountService < BaseService
+  def call(url)
+    atom_url, body = FetchAtomService.new.(url)
+
+    return nil if atom_url.nil?
+    return process_atom(atom_url, body)
+  end
+
+  private
+
+  def process_atom(url, body)
+    url_parts = Addressable::URI.parse(url)
+    username  = xml.at_xpath('//xmlns:author/xmlns:name').try(:content)
+    domain    = url_parts.host
+
+    return nil if username.nil?
+
+    Rails.logger.debug "Going to webfinger #{username}@#{domain}"
+
+    return FollowRemoteAccountService.new.("#{username}@#{domain}")
+  end
+end