add responsive sidebar to issues, milestones and MRs

app/assets/javascripts/sidebar.js.coffee

app/assets/stylesheets/generic/sidebar.scss

app/views/dashboard/issues.html.haml

app/views/dashboard/merge_requests.html.haml

app/views/dashboard/show.html.haml

app/views/groups/issues.html.haml

app/views/groups/merge_requests.html.haml

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

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

app/views/projects/milestones/index.html.haml

app/views/shared/_filter.html.haml

app/views/shared/_project_filter.html.haml