Clarify rule on button colors in panel headers

[abought]

The OSF style guide calls for buttons to be colored according to function: https://centerforopenscience.github.io/osf-style/style.html#buttons

However, buttons located in panel headers are not colored (see "components" widget in project dashboard).

The style guide section on panels would benefit from clarifying language about:

• What buttons are appropriate to place in a panel header
• Whether buttons in panel headers should be colored (default rule), or appear as grey (position of the button overriding the normal coloring rules)

https://centerforopenscience.github.io/osf-style/style.html#panels

Per discussion with @caneruguz .