Tweak Maniphest CSS, fix remarkup in description change views

Summary:
Various CSS tweaks and fixes:

• Add remarkup styling to description change views, missed this before.
• Fix CSS so that transactions with only one item (e.g., changed priority)

don't have weird floater underneath them.

• Add more space between transaction items.
• Make default background color lighter and less heavy.
• Use beigey color for comment form in Maniphest.
• Share more CSS between Maniphest and Differential (previews, feedback).
• Move "Leap Into Action" call to Differential, replace Maniphest with

thematically-consistent "Weigh In" (obviously, Maniphest has a nautical theme).

Test Plan:
Browsed Maniphest and Differential in a couple browsers, styling all seems
correct.

Reviewed By: tomo
Reviewers: tomo, aran, jungejason, tuomaspelkonen
CC: anjali, aran, tomo
Differential Revision: 328