Extract compose_state.js.

This is mostly just moving methods out of compose.js.

The variable `is_composing_message`, which isn't a boolean, has
been renamed to `message_type`, and there are new functions
set_message_type() and get_message_type() that wrap it.

This commit removes some shims related to the global variable
`compose_state`; now, `compose_state` is a typical global
variable with a 1:1 relationship with the module by the same
name.

The new module has 100% line coverage, most of it coming
via the tests on compose_actions.js.  (The methods here are
super simple, so it's a good thing that the tests are somewhat
integrated with a higher layer.)

zproject/settings.py

@@ -852,6 +852,7 @@ JS_SPECS = {
             'js/fenced_code.js',
             'js/echo.js',
             'js/socket.js',
+            'js/compose_state.js',
             'js/compose_actions.js',
             'js/compose.js',
             'js/stream_color.js',