Add a Server Log view for viewing/filtering the log in the admin UI.

This change introduces a view intended for the admin UI that displays
a log file generated by the Python logging code (using the default line
format in in djblets.log.DEFAULT_LINE_FORMAT) and allows for filtering
by log level and date. It also allows for sorting by date, to see the oldest
or most recent items at the top.

Tested each filter and sorting method in combination with the others.

Tested with logs containing a variety of log levels, and tested with multi-line log entries.