We use selectize for the related object selector widgets in the admin

UI. The version we were using was pretty old, and we had vendored it

into our source tree.
This change updates Djblets to include "@selectize/selectize" as a

dependency, and pull it in from node_modules instead of vendoring it

into our source tree.
The newer version required a couple small updates to the way we use it:

The related object selector renders values as blank because we

  actually show the selected values in a list or table below the input.

  We used to just return an empty string but internally, selectize was

  trying to do DOM manipulation on this. We now return an empty <span>

  element instead.
We need to pass mode: multivalued into the selectize options so it

  doesn't try to hide the <input> element after selecting a value.

I've also fixed the padding on <td> elements within the selected

items, because we had an admin form style that was setting that to a
There's one more visual fix for the user selector that will come in a

Review Board change (this was looking bad even with the old version of

selectize).

Built static media.
Ran js-tests.
Tested the related user, group, and repo selectors within the Review

  Board admin UI.