Add the server log view to Review Board.

This change hooks Djblets's new Server Log view in to the Review Board
admin UI. This is accessible from the admin dashboard, listed immediately
below the Server Cache entry.

To slightly improve our logging, we also now log the PID of the Review Board
process, so the many "Log file for Review Board" log lines won't seem
as redundant.

Tested along with the Djblets change. See /r/1026