Dropdown navigation is not keyboard accessible

[Tajana-Somogyi]

• [x] The dropdown should open when has keyboard focus is on "Who we are" / "About us" when fixed, and Enter/Space is pressed.
• [x] The dropdown should close:
• on Esc
• when clicking with mouse elsewhere
• when repeating Esc/Space and the focus is on "Who we are" / "About us"
• [x] All links in the dropdown should be reachable with tab key.
• [x] When tabbing after the last link in the dropdown, the focus shifts to the next main navigation item—"Case studies"