Final changes before first release

[cameron-toy]

Addresses Issues #120, #117, #123

Major packaging and architecture changes, as well as QOL and interaction improvements.

• Extension is separated into three packages: api, for core commenting components, notebook, for notebook commenting components, and text for text commenting components.
• The functionality of the Monolithic CommentFactory has been separated among CommentWidgetFactory, CommentFactory, and CommentWidget. This should give a clearer mental model of the architecture and make extension easier for other developers.
• Tighter coupling between comment files and source files. Deleting, saving, or renaming a file with comments now does the same to its associated comment file, even if it's not currently open.
• Removed separate MockCommentWidget and added isMock options to regular CommentWidgets.
• Updated @jupyterlab components
• Added context menu delete/edit/rename options to comments.
• Added context menu option for commenting on text files.
• Various small fixes.