Webtiles: add janitor_commands

This implements a system for janitors to run pre-defined commands
through webtiles. The intended use case is for crawl-developers to
perform some behind-the-scenes administrative tasks, such as:
* Kicking off a manual trunk rebuild
* Downloading save games of users that help replicate bugs

The current implementation is experimental: all janitor commands are
run in 'no-op' mode.