Reply fallback is included in edited message m.new_content

Currently, if you reply to a message, then edit the reply with RiotX, the edit event's `m.new_content` body fields contain the reply fallback. This reply fallback should not be included, as per [MSC1849](https://github.com/matrix-org/matrix-doc/pull/1849):

> The edited reply should ditch the fallback representation of the reply itself however from `m.new_content` (specifically the `` tag in the HTML, and the chevron prefixed text in the plaintext which we don't know whether to parse as we don't know whether this is a reply or not), as we can assume that any client which can handle edits can also display replies natively.

Riot web already behaves correctly: vector-im/riot-web#11129 / matrix-org/matrix-react-sdk#3551.

Additionally, since RiotX doesn't support rendering replies (#174), the reply indication disappears if someone edits a reply message from Riot web. It could be solved by hackily re-adding the fallback, but really just implementing reply rendering would be better.