Move the fleet management automation into a subdirectory

[axch]

instead of having it spread out at the top level the way it is. This may require some care, as some of the scripts may access resources by paths relative to their own location on the file system.

• [ ] aws-spend.sh
• [ ] fetch-logs.sh
• [ ] fleet.sh
• [ ] get.sh
• [ ] hostkey.sh
• [ ] login.sh
• [ ] put.sh
• [ ] running-cost-table.jq
• [ ] running-stacks.org
• [ ] set-hostname.sh
• [ ] set-jupyter-password.sh
• [ ] stack-change-set.sh
• [ ] stack-delete.sh
• [ ] stack-finish.sh
• [ ] stack.sh
• [ ] stack-start.sh
• [ ] url-cards.sh
• [ ] write-jupyter-passwords.sh