Allow creating "free text" taxonomies in the taxonomy UI

[bradenmacdonald]

Next to the import Button, add a "New ⌄ " button that opens a dropdown menu with only one option: "Free text taxonomy". Clicking that should prompt for the taxonomy name, then create an empty free text taxonomy.

See https://github.com/openedx/openedx-learning/blob/8e71a58ba4d6cb498a1adf78b9f323d50cb4bfcb/openedx_tagging/core/tagging/rest_api/v1/views.py#L109-L118 .