Release post: reference buttons - support arrays

Make the reference buttons (for issues and MRs) to support an array:

issue_url: ['issue#1','issue#2',...,'issue#n'] 
mr_url: ['mr#1','mr#2',...,'mr#n']

For that, we'll need a logic to make a normal button when there's only one link, and a dropdown menu when there's more than one link. Something like:

if top_feature.issue_url > 1
  <dropdown menu>
  "Issues"
else
  <normal link>
  "Issue"