restore explicit imports of jquery, etc.

Downgrade to CodeMirror 5.16

Closes https://github.com/jupyter/notebook/issues/1967

Add 4.3.1 to changelog

rename log.warn to log.warning as log.warn is deprecated

Use attachment:path instead of attachment://path

Further highlight token info in log output

if token came from config, assume user knows what it is
or it's handled some other way.

on login page

Ignore resize events that bubbled up and didn't come from window.

add missing waits for output in display_id tests

add critical-level log statement at the end of startup with token info

See https://bugs.jquery.com/ticket/9841. In our case, the OutputArea was triggering a ‘resize’ event on its element, which was bubbling up and causing this handler to execute every time an output was appended. This was a pretty big drain on output areas that quickly changed (like for interact widgets), presumably since this function involves a DOM read to get heights.

fix carriage return handling

The only real fix is an errant `+` on the final replacement, which would end up skipping sequential replacements.

- leaves trailing `\r` on the text, if there is one
- use groups to avoid unnecessary replace calls (no change)
- includes test

add Authorization to allowed CORS headers

so that CORS requests can be token-authenticated

better docs for token auth

This partly reverts #1659, see also #1655.

Make the font size more robust against fickle browser values.

Add output callback overrides

This allows us to override output callbacks to redirect output messages, and is used to implement the Output widget, for example. It does not redirect status messages or messages on other channels.

Basically, message processing did not wait for any promises a comm message handler may return to resolve, because comms were not set up to resolve return values of message handlers.

Add 4.3 to changelog

Also change the line-height multiplier to 1.3, which is closer to what the CSS is (I think it is 1.21 something.)

Thanks to @williamstein and @haraldschilly for bringing this to our attention: https://github.com/sagemathinc/smc/issues/1262

We also re-enable auto scrolling for Firefox, since this might have been the issue.