Refactor issue and merge request sidebar

app/assets/javascripts/dispatcher.js.coffee

app/assets/javascripts/issuable_context.js.coffee

app/assets/javascripts/issue.js.coffee

app/assets/javascripts/merge_request.js.coffee

app/helpers/gitlab_routing_helper.rb

app/views/projects/issues/_discussion.html.haml

app/views/projects/issues/_issue_context.html.haml

app/views/projects/issues/show.html.haml

app/views/projects/issues/update.js.haml

app/views/projects/merge_requests/_discussion.html.haml

app/views/projects/merge_requests/_show.html.haml

app/views/projects/merge_requests/update.js.haml

app/views/shared/issuable/_context.html.haml