Refactor local nav and make `inner` wrappers optional

[axelboc]

Fixes #773 - inner wrappers can now be omitted in the local nav markup, in which case they are injected automatically. Note that this does have a very small impact on performance, so if you're able to provide the wrappers yourself, please do so.

This PR also includes a complete refactoring of the LocalNav component. Below is a non-exhaustive list of the changes:

• use button rather than span for close/back buttons
• add BEM classes on the fly in order to clean-up the CSS and reduce the number of overridden styles
• make nested panels look the same as the root panel
• underline links on hover
• move event bindings for local nav elements out of the InjectedNav component and into the LocalNav component (pass required functions as props)
• split local nav code into separate methods to improve code readability
• add loads of comments
• fix deeply nested panels with regard to vertical overflow, by keeping track of which panels are open at any given time
• add nested panels to the doc site's localnav for testing purposes (commented-out)

[axelboc]

Turns out this fixes #679 as well 😄