Log rotation not working correctly

[dpmatthews]

The log rotation implemented in #376 appears to be flawed.
Logic was added which creates log as a symlink to 01-uiserver.log (in ~/.cylc/uiserver/log/) which is rotated at each start up.
So, we end up with a directory containing something like this:

01-uiserver.log
02-uiserver.log
03-uiserver.log
04-uiserver.log
05-uiserver.log
log -> /home/user/.cylc/uiserver/log/01-uiserver.log

However, the traitlets logging config is setup to use the file log with log rotation based on file size.
Therefore, if the log file gets large enough to trigger rotation (tested by reducing the size limit) we get this:

01-uiserver.log
02-uiserver.log
03-uiserver.log
04-uiserver.log
05-uiserver.log
log
log.1 -> /home/user/.cylc/uiserver/log/01-uiserver.log

After this, the log rotation at startup no longer works and output just gets appended to the file log.

[dpmatthews]

Note: rotating the log file at start up is, perhaps, nice but certainly not essential. Rather than implementing our own code to do this I wonder if if might be better to just go back to using the built in logging support and accept a log file which only gets rotated based on size.