Conversation block: translate default participants

https://github.com/Automattic/jetpack/pull/17916#discussion_r544212748 and https://github.com/Automattic/jetpack/pull/17916#discussion_r542285520

- Need to refactor the default participants in the block, so that they can be translated.
- As part of this, remove the `participants.json` file
- Translate `onAdd` text: https://github.com/Automattic/jetpack/commit/20b5509757ced91864f525dbc2ad1c82ff27b172#diff-e03e8c02abb0c7414076fa653fbb790e0217a664c16e1b9bbba1c9afc23a016eR14
- Use real names for defaults, rather than "Participant 1", etc

May be able to use `wp_localize_script`

Start with the conversation block's default participant data and the onAdd text referenced in the linked commit, then inspect participants.json. Check the linked pull-request discussions for the intended refactor and whether wp_localize_script is suitable. Done means participants.json is removed, default names are real names, and both participant labels and onAdd text can be translated.

Written by the indexing model from the issue text.