[6.x] Alert component

[JayGeorge]

Description of the Problem

Currently, there's no alert component

What this PR Does

• Adds an alert component
• The alert component can be inline or a more detailed message
• There are 4 variations
  • Default
  • Success
  • Error
  • Warning
• Each variation defaults to a default icon but the user can also set a custom icon

How to Reproduce

1. It's easiest to check out cp/playground, where you'll find various alert demos, shown in the screenshot above.
2. Alternatively, you can check out the commit Dashboard demo and go to cp/dashboard to see the component in situ.