Hide weight if only one column selected

[mccalluc]

• Fix #136

Shiny doesn't seem to support disabling select controls. Not in the docs, and just adding a kwarg doesn't work. Adding a note after the control follows the pattern we have with disabled buttons. For both of these styling it as a bootstrap alert might be appropriate.

For reviewer:

• Does this message seem worthwhile? or maybe it's a distraction if we can't easily disable the select?
• Would styling this message as an alert be appropriate? or would that call too much attention to it?

On the whole I think this is a good change, but I'm not totally confident about it. Feedback welcome.

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Update: Instead of adding a message, just use a hidden div when there is only one column selected. PR retitled.

[mccalluc]

@ekraffmiller : Good thoughts. I tend to shy away from controls that are disabled based on some other criterion. We've all been in the place where we can see the option we need, and it's so close, but it's grey for no apparent reason.

I'm going to try a new PR, but instead of adding a message, just wrapping the control in a hidden div. Stay tuned...