Taxonomy: Add search bar to taxonomy picker

Some projects use a very large taxonomy tree. It can be difficult for editors to find the key they want to apply to a document by browsing through the tree.

It would be desirable in these cases, for the taxonomy picker to have a search bar just above the taxonomy tree, much like the documents search bar above the documents tree.

The editors could enter a partial key/label and the tree could be reduced to only show the taxonomy items that match. Ideally the tree would be updated on key press, but it would also work if they would have to press enter to update the tree.

A button to clear the search would also be needed (like with the documents search).

See attached image or: https://bloomreach.invisionapp.com/share/V3VG8IOAMZC#/screens/399844409 for design