Rename document/image fields HTML classes

Using the `document` or `documents` classes meant styles defined for the
public list of documents conflict with these ones.

So now we're using HTML classes that match the name of the Ruby
component classes, as we usually do.

spec/support/common_actions/documents.rb

@@ -9,7 +9,7 @@ module Documents
   def documentable_attach_new_file(path, success = true)
     click_link "Add new document"
 
-    document = all(".document").last
+    document = all(".document-fields").last
     attach_file "Choose document", path
 
     within document do
@@ -22,7 +22,7 @@ module Documents
   end
 
   def expect_document_has_title(index, title)
-    document = all(".document")[index]
+    document = all(".document-fields")[index]
 
     within document do
       expect(find("input[name$='[title]']").value).to eq title