[EE] Create position-sticky scss mixin

What does this MR do?

To support Safari, position: sticky needs to be used with position: -webkit-sticky;. This MR adds a mixin to encapsulate this coupling.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Tested in all supported browsers