Add basic version of estimate slash command.

app/services/slash_commands/interpret_service.rb

@@ -248,6 +248,15 @@ module SlashCommands
     params '@user'
     command :cc
 
+    desc 'Set estimate'
+    params 'e.g: 3h 30m'
+    condition do
+      issuable.persisted? &&
+        current_user.can?(:"update_#{issuable.to_ability_name}", issuable)
+    end
+    command :estimate do |raw_duration|
+    end
+
     def find_label_ids(labels_param)
       label_ids_by_reference = extract_references(labels_param, :label).map(&:id)
       labels_ids_by_name = LabelsFinder.new(current_user, project_id: project.id, name: labels_param.split).execute.select(:id)