Allow noncritical blocks in email bodies to be disabled via config

Summary:
See T931. LLVM users would also prefer simpler mail, and have similarly harsh opinions about the current state of affairs.

Allow installs to disable the hint blocks if they don't want them.

Test Plan: Sent myself mail with these settings on/off, got mail with/without the blocks.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T931

Differential Revision: https://secure.phabricator.com/D2968