Date picker: Update keyboard navigation

Problem

• Related to gitlab-org/gitlab-services/design.gitlab.com#1142 (closed)
• Currently, we can't access the month arrows or MM / YYYY dropdowns in date picker component via keyboard
• When navigating up and down on the dates, the screen also scrolls

Proposal

1. Update tab sequence to something similar to this accessible date picker:

If gitlab-org/gitlab-services/design.gitlab.com#1142 (closed) goes ahead...

• Tab 1 = highlight text input
• Tab 2 = highlight month-back-arrow button [hit enter to go to previous month]
• Tab 3 = highlight month dropdown [hit enter to activate, up/down arrows to navigate months, enter to select month]
• Tab 4 = highlight year dropdown [hit enter to activate, up/down arrows to navigate years, enter to select year]
• Tab 5 = highlight month-forward-arrow button [hit enter to go to next month]
• Tab 6 = highlight today's date in the calendar [arrows to navigate days/months and enter to submit]
• Tab 7 = exits date picker without selecting date

If not...

• Tab 1 = highlight text input
• Tab 2 = highlight calendar button [hit enter to open calendar]
• Tab 3 = highlight month-back-arrow button [hit enter to go to previous month]
• Tab 4 = highlight month dropdown [hit enter to activate, up/down arrows to navigate months, enter to select month]
• Tab 5 = highlight year dropdown [hit enter to activate, up/down arrows to navigate years, enter to select year]
• Tab 6 = highlight month-forward-arrow button [hit enter to go to next month]
• Tab 7 = highlight today's date in the calendar [arrows to navigate days/months and enter to submit]
• Tab 8 = exits date picker without selecting date

2. Stop the scrolling when moving up and down on the date picker days