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

This feature currently lacks:
* Non-blocking implementation (for large files / long-running scripts)
* Dynamic user UI for filling in janitor_command params
* Simplified rewrite