Create new interface for labels

Problem/Motivation

The organization I work for uses labels as the main searching and filtering of assets. It is how it is organized as if it were a folder structure. The current UI make it quite difficult for us to use the labels to search for the assets within brandfolder.

Also, many of the labels we have use the same name. For example:

- 1
-- A
-- B
-- C
- 2
-- A
-- B
-- C
...
- 100
-- A
-- B
-- C

When filtering for Label "A" within "1" we get results from all labels "A" from 1 to 100

Proposed resolution

To create a simplified UI only for the labels where users can choose to use it when picking the assets.
This new UI would use the label ID to fetch the labels.

Also, for better UI we can use js_tree to be able to navigate and collapse and expand the hierarchy.

An UI inspired on the media directories module.

Given the current limitation for the label endpoint where the search parameters do not work, I've left out the search field from this new UI for now.

User interface changes

Added a new entity browser widget, so users can choose as an option.

API changes

Makes a request to the labels endpoint instead of using the search parameter on the brandfolder endpoint.