Using PinnableElement in the TOC allows us to reuse styles associated with all PinnableElements, and helps ensure consistency between page tools and the TOC. This is something that would probably need to be done for T324505 anyway
Rather than updating PinnableElement to accept a class prop, I chose to rename '.sidebar-toc' to '.vector-toc', making the TOC's naming conventions consistent with the rest of Vector.
"sidebar" doesn't make sense anymore anyway, as the TOC can be in multiple locations and it's best to avoid the "sidebar" term
Bug: T324877
Change-Id: I5b9228380f5c4674ef424d33127a5cb4010822da