Plaintext representation of nested rich replies get mangled at some point

[non-Jedi]

The plaintext representation of the rich reply example from #5969 ended up
looking like this:

&Adam:
> TravisR:
> Welcome Back Bot:
> Welcome back, @richvdh:sw1v.org

beep

So what does a reply to a reply look like?

Oh boy...

This isn't at all helpful to clients that aren't html-aware. Maybe it's only the
first message that gets > style quotes. Maybe it's specific to m.notice
messages. Either way, it's confusing.

[lampholder]

I suggest the plaintext include the message being replied to (in a predictable format), a link to that message's event, and the message text of the reply. Something like:

2017-09-15T12:04:33Z <toml> (@tom:lant.uk): Hey, bob, how's it going? [https://matrix.to/#/!lbPPsPcefsfdsefPjRn:lant.uk/$1513243242101Udovh:lant.uk]
Pretty good thanks, and you?

[t3chguy]

@lampholder how about nested in that case?

[t3chguy]

So if we want to fix #5968, then the plaintext rep wouldn't have the entire nested depth, which is going to be hard as we'll have to parse out any quoted events before quoting it ourselves

[t3chguy]

Related https://docs.google.com/document/d/1KLdKtuZBbFoWDSfN4KM3p7LnIvFBQfSORICBo8zRHaE/edit

[t3chguy]

So fwiw this issue is caused by https://github.com/matrix-org/matrix-react-sdk/blob/develop/src/components/views/rooms/MessageComposerInput.js#L853 assuming that there are no newlines. It should instead prepend > to each line in contentText

[t3chguy]

Fixed by merged PR