[Feature request] Implement bookmark search API endpoint with tag facet data

[pspanja]

Basically same as /api/bookmarks/ now works, but with information about available and selected tags for the search. Something like:

{
  "count": 123,
  "next": "http://127.0.0.1:8000/api/bookmarks/?limit=100&offset=100&q=example",
  "previous": null,
  "results": [
    {
      "id": 1,
      "url": "https://example.com",
      "title": "Example title",
      "description": "Example description",
      "notes": "Example notes",
      "website_title": "Website title",
      "website_description": "Website description",
      "is_archived": false,
      "unread": false,
      "shared": false,
      "tag_names": [
        "tag1",
        "tag2"
      ],
      "date_added": "2020-09-26T09:46:23.006313Z",
      "date_modified": "2020-09-26T16:01:14.275335Z"
    },
    ...
  ],
  "selected_tag_names": ["one", "two", "three"],
  "available_tag_names": ["four", "five", "six"],
}

This would enable implementing richer SPA search clients for example.