Added accessibility fixes for DateInput component

[Mil4n0r]

Checklist (Check off all the items before submitting)

• [x] Build process is done without errors. All tests pass in the /lib directory.
• [x] Self-reviewed the code before submitting.
• [ ] Meets accessibility standards.
• [x] Added/updated documentation to /website as needed.
• [x] Added/updated tests as needed.

Description There were some accessibility issues regarding the incorrect use of aria-selected in a button, inappropiate roles inside the Calendar, etc.

IMPORTANT: This task required modifying the existing structure of the component so I would highly recommend to wait for the next release to merge it. Feel free to review it though.

Closes #1944, closes #1666

[Mil4n0r]

Closed for now until the topic is discussed.

[Mil4n0r]

Reopened.

[Mil4n0r]

Updated, now also fixes #1666