Support loading messages for a given thread

[nesium]

When displaying a message that is the beginning of or belongs to a thread we need a method to load all other messages from that thread from the server.

Ideally when scrolling through a conversation, messages belonging to a thread would not be returned from MAM unless the parent message is selected to disclose the thread.

Refs:

• Support fetching fastenings for a specific message #10
• Support opening a message from a deep-link prose-app-web#56
• https://xmpp.org/extensions/xep-0201.html