Add helper method #time_spent to Issuables.

This method returns the total spent time that the user has declared
through the `/spend` slash command.

app/models/concerns/issuable.rb

@@ -303,4 +303,8 @@ module Issuable
     metrics = self.metrics || create_metrics
     metrics.record!
   end
+
+  def time_spent
+    timelogs.sum(:time_spent)
+  end
 end

app/services/slash_commands/interpret_service.rb

@@ -255,7 +255,7 @@ module SlashCommands
     end
     command :estimate do |raw_duration|
       begin
-        @updates[:estimate] = ChronicDuration.parse(raw_duration, default_unit: 'hours')
+        @updates[:time_estimate] = ChronicDuration.parse(raw_duration, default_unit: 'hours')
       rescue ChronicDuration::DurationParseError
         # do nothing
       end