changes tags to a list of dicts in the db, adding tag slugs

- adds a function to convert the tag list of dicts to a text string
  properly delimited for loading into forms
- tag string conversion function updated to generate list of dicts
- updates all mentions of the conversion of the string to the tags db
  object
- adds a tags template utility and updates the media template accordingly

mediagoblin/db/models.py

@@ -82,7 +82,7 @@ class MediaEntry(Document):
         'media_type': unicode,
         'media_data': dict, # extra data relevant to this media_type
         'plugin_data': dict, # plugins can dump stuff here.
-        'tags': [unicode],
+        'tags': [dict],
         'state': unicode,
 
         # For now let's assume there can only be one main file queued