Cleaned up FactoryGirl calls for object lists.

For each of the "strategy" methods FactoryGirl implements, such as
#build, #attributes_for, etc., there is a corresponding method to build
lists of items: #build_list, #attributes_for_list, etc. The
documentation, especially on a quick read, points out `build_list` and
`create_list` without noting that *any* strategy method has such a list
method. This is Useful, allowing us to clean up some code and speed up
specs slightly.

RSpec: 581 examples, 0 failures
RuboCop: 139 files inspected, no offenses detected