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

scripts/celerity/generate_sprites.php

@@ -227,6 +227,31 @@ foreach ($action_map as $icon => $source) {
       ->setTargetCSS('.action-'.$icon));
 }
 
+
+$remarkup_template = id(new PhutilSprite())
+  ->setSourcePosition(0, 0)
+  ->setSourceSize(14, 14);
+
+$remarkup_icons = array(
+  'b',
+  'code',
+  'i',
+  'image',
+  'ol',
+  'tag',
+  'tt',
+  'ul',
+  'help',
+);
+
+foreach ($remarkup_icons as $icon) {
+  $sheet->addSprite(
+    id(clone $remarkup_template)
+      ->setSourceFile($srcroot.'remarkup/text_'.$icon.'.png')
+      ->setTargetCSS('.remarkup-assist-'.$icon));
+}
+
+
 $sheet->generateImage($webroot.'/image/autosprite.png');
 $sheet->generateCSS($webroot.'/css/autosprite.css');