Add buy pipelines link subtext

What does this MR do?

Add warning subtext to Buy Pipeline minutes link in user dropdown as described below

• Add One of your groups is running out with color $orange-800 - #a35200
• Whole area of the "Buy Pipeline minutes" is clickable, including the warning message below it.

Note: This is behind an experiment/feature flag, so no changelog is provided.

Screenshots

Showing entire section is part of the clickable link:

Screen_Recording_2020-06-11_at_10.14.08_AM

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Contributes to #221063