Small improvements to the web-based log viewer

[alard]

Some cosmetic improvements to the web interface.

For the logs:

• This improves the handling of carriage returns (semi-broken since Fix performance issue with large item logs #99) for outputs like the rsync upload progress.
• Long log files are trimmed in the middle, keeping the first and last lines, and adds a message at the trimming point.
• Null characters (such as for the reddit task) are replaced by spaces.

The CSS overflow for long item titles is hidden (again inspired by the long item names for reddit).