Implement improved remarkup assistance panel

Summary:

• I made a "?" icon for help/reference.
• The <> icon was slightly too wide so I carved it down to 14x14.
• All the icons are in /Phabriactor/remarkup_icon_sources.psd if you want to tweak anything.
• Tooltips don't look like the mock but I'll tackle those separately.
• Removed strikethrough.
• Removed tag/image/text size for now since they don't have reasonable JS implementations yet.
• I think everything else is accurate to the mock.

Test Plan:
Normal state:

{F20621, size=full}

Hover + Click states:

{F20622, size=full}

Clicked state:

{F20620, size=full}

Reviewers: chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T1848

Differential Revision: https://secure.phabricator.com/D3650