Add footnotes support to Markdown

Signed-off-by: Guillaume Delbergue <guillaume.delbergue@greensocs.com>

CHANGELOG

@@ -17,6 +17,7 @@ v 7.11.0 (unreleased)
   - Improve UI for sidebar. Increase separation between navigation and content
   - Improve new project command options (Ben Bodenmiller)
   - Prevent sending empty messages to HipChat (Chulki Lee)
+  - Add footnotes support to Markdown (Guillaume Delbergue)
 
 v 7.10.0 (unreleased)
   - Ignore submodules that are defined in .gitmodules but are checked in as directories.

app/helpers/gitlab_markdown_helper.rb

@@ -49,7 +49,8 @@ module GitlabMarkdownHelper
         strikethrough:       true,
         lax_spacing:         true,
         space_after_headers: true,
-        superscript:         true
+        superscript:         true,
+        footnotes:           true
       )
     end