Keyboard shortcut to move cells

The move cell up/down feature is useful. It would be even more useful if it could be done quickly from the keyboard. I think Alt + Up/Down would be a good shortcut for this, and it's a broad enough use case where most people would find it useful.

The issue names no files, tests, or entry points. Start by locating the notebook UI's existing move-cell up/down action and keyboard shortcut definitions. Done means Alt+Up and Alt+Down move the selected cell in the corresponding direction without interfering with other shortcuts.

Written by the indexing model from the issue text.