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:
* Streaming stdout / return-code data for executed commands (the command
  is daemonised and the janitor can't get any data from it)
* Dynamic user UI for filling in janitor_command params
* Refactoring/simplification