Move sponsor modal to a generic partial & redesign.

... use for booths also.

app/views/conferences/_modal_description.haml

@@ -0,0 +1,21 @@
+- content_for :modals do
+  .modal.fade{ id: "modal-#{object.class.name.downcase}-#{object.id}" }
+    .modal-dialog
+      .modal-content
+        .modal-header
+          %button.close{ data: { dismiss: 'modal' } }
+            %i.fa.fa-close
+          %h3.modal-title
+            = object.try(:title) || object.try(:name)
+        .modal-body
+          %p.logo
+            = link_to object.website_url || '#', target: '_blank' do
+              - img_classes = ['img-responsive center-block']
+              - if object.is_a? Sponsor
+                - img_classes << ['img-sponsor',
+                  "img-sponsor-#{object.sponsorship_level.position}"]
+              = image_tag get_logo(object), class: img_classes
+          %p.description
+            = object.description
+        .modal-footer
+          = link_to nil, object.website_url, target: '_blank'