Replying to reply is broken

[notramo]

Replies contain fallback quoting of original message. Element filters these fallback texts out, so only the last message is quoted. Syphon doesn't filter out the reply quotes, so it gets unreadable garbage with a long reply chain.

Steps to reproduce

1. Send a message
2. Reply to that message
3. Reply to the message sent in step 2.

If the message in step 3 was sent from Element, it only contains step 2 as a quote. If it's from Syphon, then step 1 and step 2 are quoted, with wrong formatting.

How to fix it?

When inserting fallback quotes:

1. Filter out the first Markdown blockquote in unformatted text
2. Filter out text inside <mx-reply> tag from the start of formatted_text

[EdGeraghty]

This is related to #529