show unlock date for files and pages in locked modules

test plan:
* create a module
* add a wiki page and an attachment as module items
* set the module to unlock on a future date
* try to visit the page or file as a student
* should show the unlock date in the message

closes #CNVS-6681

Change-Id: Id311b029bc8bfe7c087550ca233a40b4f6d3c543
Reviewed-on: https://gerrit.instructure.com/22190
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>

app/models/attachment.rb

@@ -1201,6 +1201,7 @@ class Attachment < ActiveRecord::Base
         locked = {:asset_string => self.asset_string, :lock_at => self.lock_at}
       elsif self.could_be_locked && item = locked_by_module_item?(user, opts[:deep_check_if_needed])
         locked = {:asset_string => self.asset_string, :context_module => item.context_module.attributes}
+        locked[:unlock_at] = locked[:context_module]["unlock_at"] if locked[:context_module]["unlock_at"]
       end
       locked
     end

app/models/wiki_page.rb

@@ -198,6 +198,7 @@ class WikiPage < ActiveRecord::Base
       locked = false
       if (m && !m.available_for?(user))
         locked = {:asset_string => self.asset_string, :context_module => m.attributes}
+        locked[:unlock_at] = locked[:context_module]["unlock_at"] if locked[:context_module]["unlock_at"]
       end
       locked
     end

lib/has_content_tags.rb

@@ -43,7 +43,7 @@ module HasContentTags
   end
   
   def locked_cache_key(user)
-    ['_locked_for', self, user].cache_key
+    ['_locked_for2', self, user].cache_key
   end
   
   def clear_locked_cache(user)