widgets of Admin Dashboard do not reflect changes

Fixed Issue 2753 (Some Admin Dashboard widgets do not reflect changes to the data they contain).
Admin Dashboard widgets did not reflect when we add the repository because Admin Dashboard could not detect changing in this cache form.
So I change form of cache, I add username and syncnum which is value for invalid old caches.

[reviewboard/admin/__init__.py]
	* bring post_save.connect and post_delete from signals.py.
[reviewboard/admin/widgets.py]
	* add username and syncnum, is value for invalid old caches, to generate_cache_key().
[reviewboard/admin/signals.py]
	* change _delete_widget_cache() to increment syncnum.
	* remove post_save.connect and post_delete.

I check the Admin Dashboard after add a repository, then I can see correct data on Repositories widget.