Fixed hover of date picker calendar

What does this MR do?

Fixes the hover styling of days on the datepicker dropdown in the issuable sidebar.

Screenshots (if relevant)

Unmarked this merge request as a Work In Progress

Reassigned to @jschatz1

Thank goodness @iamphill You rock!

Status changed to merged

mentioned in commit cf879c93

Removed ~149423 label

Picked into 8-9-stable, will be in RC8.

mentioned in commit d9bf70b9