Fix clearing url hash on details close.

- Update the entity-base to pass a null hash in the metadata during the change
event.
- Add tests to both the bundle and charm details that the hash is cleared.

QA
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Make sure to use the :flags:/il for all QA steps.

 - open a charm details view, go to the readme tab. Verify the url has
   #readme. Close the details and the url should not have any hash any longer.
 - Repeat the process, make sure that you can copy and paste that url with the
   hash into a new browser tab. Then close the details, and the hash goes away.
 - Do the same for bundles.