fix: Keyboard accessible context menus

[tommoor]

• Makes menus fully accessible and keyboard driven
• Currently adds 2.8% to initial bundle size due to the inclusion of Reakit and its dependency, popperjs.
• Converts all menus to functional components
• Remove old custom menu system
• Various layout and flow improvements around the menus

closes #1766

[tommoor]

This still isn't mergable as it's not as performant due to the menus rendering all of their contents before they are clicked, which is different to the old system. I think in order to get this feeling good the key menus that are on every page (DocumentListItem, DocumentLink, CollectionLink) will need to be rendered only when visible which will neccessitate moving hover/focus into React state.

[thenanyu]

what does the leading $ mean generally?

[tommoor]

Stops it being added to the dom by styled-components: https://styled-components.com/docs/api#transient-props