log: job summary header

[oliver-sanders]

The job log viewer page in Cylc Review showed a short summary of the job above the log file.

This was useful for:

• Checking the status of the job.
• Checking the exit signal of the job e.g. XCPU.
• Navigating to subsequent submissions of the job.
• Viewing timing information.

It would be good to build something like this back into the new log view, additional suggestions:

• Display task messages / outputs or have a button to display them.

This will require a one-off query of the job being viewed. See #1241 which also requires this query.