Bubble chart missing `size` option

[tdsmith]

Issue Summary

The bubble chart type is missing a box to specify a size variable.

Steps to Reproduce

1. Create a new visualization.
2. Choose a "chart" viz with chart type "bubble".

Any other info e.g. Why do you consider this to be a bug? What did you expect to happen instead?

Bubble charts show (x, y, size) data; there used to be a way to specify sizes I'm pretty sure!

Technical details:

• Redash Version:
• Browser/OS: Fx 74.0b5
• How did you install Redash: