Feature: Thread support fallback

[notramo]

Thread messages are structurally similar to replies, and can be displayed using the reply rendering solution (maybe with some icon to indicate it's a thread) to the previous thread messages until proper support is added.

Replies:

"m.relates_to": {
    "m.in_reply_to": {
        "event_id": "aaaaaaabbbbbbbcccccccccddddddd"
    }
}

Thread messages:

"m.relates_to": {
    "event_id": "ddddddddddeeeeeeeeefffffffffffgggggggg",
    "rel_type": "io.element.thread"
}

Fallback solution in MSC3440

[notramo]

This MSC changed since I opened this issue, and the desired fallback effect can be produced without adding any support in Cinny, so I close this issue.