Allow users to provide alt text for image embeds

Summary:
Ref T4843.

  - Add an `alt` attribute so users can provide alternate text for `{Fnnn}`.
  - Add an `alt` attribute to image macros.

Test Plan: Embedded an image with `alt` and a macro, inspected HTML source to verify the `alt` attribute was present.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4843

Differential Revision: https://secure.phabricator.com/D8925

src/applications/files/remarkup/PhabricatorRemarkupRuleEmbedFile.php

@@ -55,6 +55,7 @@ final class PhabricatorRemarkupRuleEmbedFile
       'float'   => false,
       'width'   => null,
       'height'  => null,
+      'alt' => null,
     );
 
     if ($option_string) {
@@ -112,6 +113,10 @@ final class PhabricatorRemarkupRuleEmbedFile
       }
     }
 
+    if (isset($options['alt'])) {
+      $attrs['alt'] = $options['alt'];
+    }
+
     $img = phutil_tag('img', $attrs);
 
     $embed = javelin_tag(

src/applications/macro/remarkup/PhabricatorRemarkupRuleMeme.php

@@ -36,10 +36,17 @@ final class PhabricatorRemarkupRuleMeme
         $options['src'].' <'.PhabricatorEnv::getProductionURI($uri).'>'.
         ($options['below'] != '' ? "\n\"{$options['below']}\"" : '');
     } else {
+      $alt_text = pht(
+        'Macro %s: %s %s',
+        $options['src'],
+        $options['above'],
+        $options['below']);
+
       $img = phutil_tag(
         'img',
         array(
           'src' => (string)$uri,
+          'alt' => $alt_text,
         ));
     }
 

src/docs/user/userguide/remarkup.diviner

@@ -347,7 +347,7 @@ from clipboard into the text area.
 
 You can set file display options like this:
 
-  {F123, layout=left, float, size=full}
+  {F123, layout=left, float, size=full, alt="a duckling"}
 
 Valid options are:
 
@@ -360,6 +360,7 @@ Valid options are:
     text
   - **width** Scale image to a specific width.
   - **height** Scale image to a specific height.
+  - **alt** Provide alternate text for assistive technologies.
 
 == Embedding Countdowns