cat-log: new auto cat/tail mode

[oliver-sanders]

• cylc cat-log <job-id> will cat/tail the requested log file for the specified job.
  • --mode=cat (CLI default) will cat the file.
  • --mode=tail (UIServer default) will tail the file.

Issues:

1. If the job has finished, the job log files are no longer growing and the tail operation is not required.
2. However, the UI Server will always request --mode=tail because it doesn't know any better.
3. Excess tail commands may cause load.

Solution:

• Add a new --mode called "auto" (or something of that ilk) that will tail if the job is running, and cat otherwise.