Upgrade to v3 API for tag_autocomplete

The tag autocompletion currently uses the old util API.
Changing the presets.json from:
`"data-module-source": "/api/2/util/tag/autocomplete?incomplete=?"`
to
`"data-module-source": "/api/3/action/tag_autocomplete?query=?"`
won't work because the former returns `ResultSet[Result]` and the latter returns `result`.

I see the parseCompletions code in ckan/public/base/javascript/client.js
`var raw = jQuery.isArray(data) ? data : data.ResultSet && data.ResultSet.Result || {};`
but when I change it to `raw = data.result`
it doesn't work. Any tips?

Inspect presets.json and ckan/public/base/javascript/client.js, starting with parseCompletions and the two API response shapes described in the issue. Determine the compatible handling for tag_autocomplete, then verify that tag autocompletion works with the v3 endpoint and its returned data.

Written by the indexing model from the issue text.