Combine job activity logs in a single file

Job submit, poll, kill, event hook standard output and error are now
written to the `job-activity.log` file.

Added a basic test for this change.

Fixed some documentation issues.

N.B. The style of `cylc.job_logs` is modified to allow everything to be accessible via a single symbol - the `CommandLogger` class. Some variable names (e.g. `type` and `dir`) are adjusted to avoid Python built-ins.

See also #1112 and #1115.