Warn users if markdown is going to kick-in on their message, and give a nice UI for disabling it if desired

[ara4n]

This was originally element-hq/element-web#439.

Conversely, we could warn the user that their message contains markdown, and they should turn it on when desired.

UI-wise... um.... this could be a toggleable plinth that appears at the RHS of the input box? Or at the RHS of the the status area above the input box? The latter is probably nicest. We probably want regexp-based heuristics to guess about presence of markdown rather than to run markdown on the message every time you press a key.

[AmandineLP]

A toggle on the RHS of the status area would be confusing IMO: one might wonder to what it applies. I think it should be in the text input box.

A small icon like in here would be nice, much smaller than the attachment/call ones, literally similar to the one here:

Ideally, if we had a few of them, I would have them displayed as a column before the attachment one. But with one only it would look a bit strange. The other option would be to display it above the text being typed.
In any case I agree it should be displayed only when markdown is detected.

[lampholder]

I haven't ever felt the need for this feature in a RT/MD composer world. Do we still want it?

[turt2live]

/me raises lampholder's question again: do we still want/need this?

[aaronraimist]

/me raises lampholder's question again: do we still want/need this?

Yes people still want this

[tleydxdy]

please have options to disable markdown, perhaps in the settings there can be a option to choose which to defaults to, and have both /plain and /md for temporary choosing one