caltechlibrary/cell-atlas: Release 2.0

This release adds brand new features to the "The Atlas of Bacterial & Archaeal Cell Structure" site, as well as enhancing existing features: - Adds a protein viewer to view most of the PDBs throughout the book - Adds progress bar that can be toggled on/off during reading - Adds interactive chapter summary menus at the end of each chapter - Corrected many of the entries within the "Feature Index" section of the book - Refactored underlying code for existing features - Various bugfixes/improvements to the experience of existing features - Updated offline versions to include all new features (except video scrubbing, video quality switching, and protein viewer)