User defined flowbook cells

The idea here would be that the user could define a function like

```
sendRateToBucket = (tables=<-, bucket) =>
...
```

and to have that create a user defined flowbook cell where the user could specify the bucket in a UI.

The issue does not name any files, tests, or entry points. First clarify how a user-defined function should become a Flowbook cell and how the bucket is selected in the UI; define the expected user flow and completion criteria before locating implementation areas.

Written by the indexing model from the issue text.