Document setting the keys that will send a message or create a new
line when composting a message via the compose box and via the
personal settings overlay.
Updates links and redirect from former help center article about
this feature.
Fixes#31620.
This fixes the annoying behaviour where a task title with a url in it,
for example, "Fix issue https://github/com/zulip/zulip/issues/1234"
would be wrongly split into a description at the `:` in the url.
Lauryn Menard
N-Shar-ma
David Rosa