R1:f4f3b90c871f

On tasks, put Task Graph, Mocks and Mentions into a tabgroup

Summary:
Fixes T4788. This change:

- converts the "Task Graph" into a "Related Objects" tabgroup.
- makes "Task Graph" the first tab in the group.
- moves "Mocks" to become a tab.
- adds a new "Mentions" tab, which shows inbound and outbound mentions.

Primary goal of "mocks" is to give us room for a pinboard/thumbnail view after the next Pholio iteration. Might make sense to make it the default tab (if present) at that point, too…