Add megamenu to the theme.

[petrillek]

As as user I would like to have better overlook over site structure without clicking too much.
As an editor I would like to show important links in main menu.
As a disabled user I would like the megamenu to be as accessible as possible.

Research

We have been loking for implementation of megamenu, although some were working fine from the accessibility point of view they had some issues.
There is not such component in Desing System as yet.
Finally we have found already existent, open source library with good accessibility approach.

Accessible Mega Menu from Adobe

• https://adobe-accessibility.github.io/Accessible-Mega-Menu/
• Open Source
• Accessible by default
• Flexible (supports dropdowns, megamenus with 3 levels, megamenus with item descriptions)
• Simple list structure, close to Drupal menu system