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, since mocks are probably more important than related tasks when they're present.

Primary goal of "mentions" is to provide a bit of general support for various freeform relationships between tasks: if you want to treat tasks as "siblings" or "related" or "following" or whatever, you can at least find them all in one place. I don't plan to formalize any of these weird one-off relationships in the upstream, although it's vaguely possible that some far-future update might just let you define arbitrary custom relationships and then you can do whatever you want.

Test Plan: {F1906974}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4788

Differential Revision: https://secure.phabricator.com/D16806