Make Pikaday dropdowns style consistent

What does this MR do?

Change Pikaday date picker dropdowns to new style.

This affects

• issuable sidebar
• new issue
• new merge request
• group member expiration date
• project member expiration date

Screenshots

Before	After

What are the relevant issue numbers?

#28921 (moved), #35424 (closed)

mentioned in merge request !13424 (merged)

changed the description

mentioned in issue #28921 (moved)

changed milestone to %10.0

assigned to @cperessini

@winh LGTM. Approving.

assigned to @winh

approved this merge request

assigned to @iamphill

mentioned in commit c03f7825

merged