Feature Request: show user message history (for moderators)

[JohnXLivingston]

Moderators should be able to easily find all message from a specific user.

(Asked by DavDuf)

Reference: related to NlNet-task-23

TODO:

• [x] implement the feature
• [x] implement actions on the occupant list or in the occupant modal (i'm waiting for some code on ConverseJS side)

[JohnXLivingston]

This PR implements the missing buttons: https://github.com/JohnXLivingston/peertube-plugin-livechat/pull/493