Fix bug where challenge button should not be enabled when a project is not selected

[nieldlr]

Fixes #200.

There was also an additional bug that was found, where if you deselected the project with a single select component, it did not remove the selected projectId from the parent state. See this fix in this file here: ui/src/components/Select/index.js

I hope I didn't miss anything that makes this a breaking change for other components. Do let me know if there's anything else needed here.