Solve issue #3940: The dropdown submenu at notebook page is not keyboard accessible

[taohan16]

Add "dropdown-toggle" and "dropdown-submenu" classes to link element in html.

Addresses #3940

[takluyver]

Are all the CSS & JS changes necessary for this as well, or did they just get mixed up with the HTML changes you mentioned? If they're necessary, can they have some comments to briefly explain what they are?

[taohan16]

Are all the CSS & JS changes necessary for this as well, or did they just get mixed up with the HTML changes you mentioned? If they're necessary, can they have some comments to briefly explain what they are?

For issue#3940, the change in menubar.js is to prevent the dropdown menu is closed when clicking.
The changes in notebook.html are adding "dropdown-toggle" class and data-toggle attribute to tag of submenu because of the when keyboard focus on the dropdown submenu option is actually on the element.
The changes in acc_overwrites.less is to prevent the focus highlight border on submenu options lost after changing.

[blink1073]

Thanks!