Separate the inline comment summary element into a separate view

Summary:

• Affects the "Inline Comments" summary table which appears in comments that have attached inlines in the discussion threads in Differential.
• Prepares for inclusion in Diffusion.
• No application changes (minor CSS), just factors code better.
• Simplify/separate CSS.

Test Plan: Looked at on-diff and off-diff comment summaries in Differential, display looked correct.

Reviewers: davidreuss, nh, btrahan

Reviewed By: davidreuss

CC: aran, epriestley

Maniphest Tasks: T904

Differential Revision: https://secure.phabricator.com/D1928