Implemented tasks log.

For now I am happy just leaving the other logs as a disk based mechanism.

Users with advanced sites will typically be happy enough using a unix shell.

Each log need manually setting up to start logging anyway (as described in the code book).

Actually I have added supported for viewing any .log file that is there, and clearing, deleting, or downloading them. We already did viewing for the permissions log so it wasn't a lot of code.

We can consider this done. I don't want to do auto-refresh. Manual refresh is easy enough, especially on the download links (which open a text dump that can be refreshed).