First Class Sessions

[jameskerr]

This is a ticket to document a new feature where we make the sessions abstraction front and center in the app.

There will be:

• A new sidebar section for opening previous sessions.
• A way to rename a session.

[philrz]

While written in the context of a bug was fixed by the addition of the feature, this functionality is verified in #3132 (comment).

The ability to rename Sessions as described above was not covered in the initial implementation in #2467, so I've opened a new issue #3144 as a reminder for us to get to that at some point.

Thanks @jameskerr!