Allow starring / pinning tables and explorations

[kgodey]

Problem

All of our tables and explorations have equal visual weight right now.

Most schemas involve a central table or three that you want to open regularly plus several auxiliary support ones that you almost never open.

Proposed solution

It would be helpful to add the ability to star / pin individual tables and / or explorations. I propose adding this as a global for now, and on a per-user basis later if we have user requests for it.