Make the entity browser and embed button configurable

Problem/Motivation

Currently, the entity browser is hard-coded as 'paragraph_items' and the embed button is hard-coded as 'paragraphs_inline_entity_form'.

It should be possible to configure this in the UI (or in a hook alternatively).

Proposed resolution

Add a UI to enable embed buttons and entity browser pairs for use with paragraphs_inline_entity.

Remaining tasks

* create the UI for config
* Change paragraphs_inline_entity_form_entity_browser_is_paragraph_item to use the config