Fixed output of logs (tail/maintail) in the Web UI

Because forgot to give the contents via `tail_f_producer.more()`
The original code pushed the `tail_f_producer` instance onto the request.  This was intentional because the code is asynchronous.   Once the producer instance is pushed, the event loop will call `more()` on it each time it comes around.  This is how we are able to stream log output continuously when you use `supervisorctl tail -f`.

This patch changed the code to push the result of `tail_f_producer(x).more()` instead of the producer instance.  The result of `more()` will be a string with some log output.  This is why the patch may have appeared to fix the output on the web interface.  However, this doesn't set up the producer in the event loop like we originally intended, and it can never stream this way (the `-f` in `tail -f`).