config: configure default websocket ping interval

[oliver-sanders]

• Closes #557
• Sends regular pings to websocket clients to ensure that the connection is still active and open at the other end.
• This allows for earlier detection of client-side connection closing.
• It should also circumnavigate web proxy killing of idle websockets.

To review, you might want to jam a debugger into this method:

https://github.com/tornadoweb/tornado/blob/f399f40fde0ae1b130646db783a6f79cc59231b2/tornado/websocket.py#L1540-L1557

Or mess around with the ping timeout.

Check List

• [x] I have read CONTRIBUTING.md and added my name as a Code Contributor.
• [x] Contains logically grouped changes (else tidy your branch by rebase).
• [x] Does not contain off-topic changes (use other PRs for other changes).
• [x] Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• [ ] Tests are included - tornado functionality vendored by jupyter_server, not something for us to test
• [x] CHANGES.md entry included if this is a change that can affect users
• [x] Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• [x] If this is a bug fix, PR should be raised against the relevant ?.?.x branch.