Resolve "Implement flexbox in dropdowns"...and add an IE11-specific stylesheet

What does this MR do?

• Uses flexbox to display dropdowns
• Removes all the specific height rules that only work for existing dropdowns (new ones are being added which just results in having to add more exceptions
• Adds an IE-specific stylesheet to fix a flexbox quirk when using flex-direction: column
  • Instead of doing any dynamic dropdown height calculation (which wouldn't work for dynamic content loading), we will just display all dropdowns at the max height, regardless of the content height.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/45116

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

Closes #45116 (moved)