Issue 2797 - Add "Update From Gallery" key to the Safari extension update manifest

sitescripts/extensions/template/updates.plist

 <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
 <plist version="1.0">
 <dict>
    <key>Extension Updates</key>
    <array>
      {%- for extension in extensions %}
      <dict>
        <key>CFBundleIdentifier</key>
        <string>org.adblockplus.{{ extension.basename }}</string>
        <key>Developer Identifier</key>
        <string>{{ extension.certificateID }}</string>
        <key>CFBundleVersion</key>
        <string>{{ extension.version }}</string>
        <key>CFBundleShortVersionString</key>
        <string>{{ extension.shortVersion }}</string>
        <key>URL</key>
        <string>{{ extension.updateURL }}</string>
+       {%- if extension.updatedFromGallery %}
+       <key>Update From Gallery</key>
+       <true/>
+       {%- endif %}
      </dict>
      {%- endfor %}
    </array>
 </dict>
 </plist>