Implement a tag_update() logic action function

[seanh]

So users can rename a tag and/or move it between vocabularies using the API.

Currently we have create_tag() and delete_tag(), but if you were to 'update' a tag by deleting it and then recreating it all the datasets that had that tag will have lost it and you'll have to re-add it to them all.

What should happen to datasets that have the tag, if the tag gets moved between vocabularies? All the datasets just keep the tag with the new vocabulary? This will become a problem if/when we support 'radio button'-style vocabularies (where each dataset can only have one tag from the vocabulary).