Make collapsed inlines more useful and anchor target highlights more accurate

Summary:
Ref T12616. Fixes T11648. Currently, we snug up replies with a negative margin (from T10563) but this throws off the anchor highlighting.

Instead:

• Remove padding from these dolumns.
• Use margins on the stuff inside them instead.
• Less margins for replies.
• Less margins for collapsed comments.
• Show some text for collapsed comments.

Test Plan:
{F4960890}

{F4960891}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12616, T11648

Differential Revision: https://secure.phabricator.com/D17913