Bring back the user-visible descriptions for each module in the Infusion Builder

We've gone through a few extra design iterations and decided that we want to back to the design where each module description is visible to all users. This will improve accessibility and generally make Builder easier for newcomers to understand.

This will involve two main steps:

1. Bring back the implementation that shows the descriptions for each module, as well as the associated unit tests. These were removed at r8844 and r8845
2. Bring back any necessary HTML and CSS required to display these in an awesome way.