This is intended as a pure refactor, making the data flow clearer in
preparation for further changes. In particular, this makes it
manifest that the calls to `get_recipient` and `is_valid_conversation`
don't depend on anything else that has happened during the call to
`handle_text_input`.
This is indeed a pure refactor because
* is_valid_conversation itself has no side effects, either in the
implementation in typing.js or in any reasonable implementation,
so calling it sooner doesn't affect anything else;
* if we do reach it, the only potentially-side-effecting code it's
moving before is a call to `stop_last_notification`, and that in
turn (with the existing, or any reasonable, implementation of
`notify_server_stop`) has no effect on the data consulted by
the implementation of `is_valid_conversation`.
This adds the general machinery required, and sets it up for the file
`typing_status.js` as a first use case.
Co-authored-by: Anders Kaseorg <anders@zulipchat.com>
Greg Price