Date picker - make month/year look more interactive

Problem

• The date picker has a dropdown for the month and year and nobody knows about it!
• Related to gitlab-org/gitlab-services/design.gitlab.com#601 (closed). From this thread: gitlab-org/gitlab-services/design.gitlab.com#601 (comment 689319507)

Proposal

• Use a similar pattern to the sidebar - with a tertiary button that goes to gray-50 on-hover to show interaction...

Other ideas