Use select2 dropdown for dashboard/group 'New X' buttons

app/assets/javascripts/api.js.coffee

app/assets/javascripts/project_select.js.coffee

app/assets/stylesheets/framework/common.scss

app/helpers/selects_helper.rb

app/views/dashboard/issues.html.haml

app/views/dashboard/merge_requests.html.haml

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

app/views/groups/issues.html.haml

app/views/groups/merge_requests.html.haml

app/views/shared/_new_project_item_select.html.haml