Customize tier-based callouts

[dacharyc]

Currently, we have a general callout for features that are only available on select tiers.

To provide additional visual distinction, we could add specific callouts that correspond with each pricing tier. The corresponding tier could be listed in the title, and the color could match the Tugboat style guide for each respective tier's color.

If we implement this, we'd need to change the existing callout styles for tip and info - which are green and blue, respectively - two colors that might be too similar to some of the tier-specific colors in Tugboat's brand style guide.