Unify shared styles

Just a little PR removing a bit more cruft from the Manuals-Frontend mainly:

Removing the class 'manual-subsection' from the codebase and putting all of that responsibility onto the class 'js-openable' which is a better name and also one that was already there.

Removing subsection summary styles which are also no longer used

Removing linked-title from the print stylesheet, which should have been removed sooner (in an earlier commit which removes it from the rest of the codebase)