Webui: Make opening CI logs faster

Opening large log files on Coin webui can take a lot of time that can be annoying for users that might need to wait for minutes depending on the speed of the computer they are using. If the log files would be opened faster, this would improve Coin webui's user-experience.

To reproduce:

1. Open qtbase tests log or rawlog
2. Watch a web page "Loading..." until the log is displayed

One solution could be instead of reading the entire log file that can be megabytes of size, the file could be opened incrementally.