Fix nested task lists

When nesting task list items, the parent item is wrapped in a `<p>` tag.
Update the task list parser to handle these paragraph wrappers.

app/models/concerns/taskable.rb

@@ -5,7 +5,7 @@
 # Used by MergeRequest and Issue
 module Taskable
   TASK_PATTERN_MD = /^(?<bullet> *[*-] *)\[(?<checked>[ xX])\]/.freeze
-  TASK_PATTERN_HTML = /^<li>\[(?<checked>[ xX])\]/.freeze
+  TASK_PATTERN_HTML = /^<li>(?<p_tag>\s*<p>)?\[(?<checked>[ xX])\]/.freeze
 
   # Change the state of a task list item for this Taskable.  Edit the object's
   # description by finding the nth task item and changing its checkbox

lib/gitlab/markdown.rb

@@ -352,11 +352,12 @@ module Gitlab
       # ActiveSupport::SafeBuffer, hence the `String.new`
       String.new(text).gsub(Taskable::TASK_PATTERN_HTML) do
         checked = $LAST_MATCH_INFO[:checked].downcase == 'x'
+        p_tag = $LAST_MATCH_INFO[:p_tag]
 
         if checked
-          "#{li_tag}#{checked_box}"
+          "#{li_tag}#{p_tag}#{checked_box}"
         else
-          "#{li_tag}#{unchecked_box}"
+          "#{li_tag}#{p_tag}#{unchecked_box}"
         end
       end
     end

spec/helpers/gitlab_markdown_helper_spec.rb

@@ -817,6 +817,17 @@ EOT
       )
     end
 
+    it 'should render checkboxes for nested tasks' do
+      rendered_text = markdown(@source_text_asterisk, parse_tasks: true)
+
+      expect(rendered_text).to match(
+        /<input.*checkbox.*valid unchecked nested task/
+      )
+      expect(rendered_text).to match(
+        /<input.*checkbox.*valid checked nested task/
+      )
+    end
+
     it 'should not be confused by whitespace before bullets' do
       rendered_text_asterisk = markdown(@source_text_asterisk,
                                         parse_tasks: true)