디자인 이미지(배경이미지) 구현.
DesignImage Routes에 Path명 적용
DesignImage / User 관계 설정 및 이미지 보여주기
Merge branch 'master' into design

Conflicts:
	app/views/users/show.html.slim
	db/schema.rb
BackgroundImage HelperMethod 추가 / BackgroundImage 표시
Merge branch 'master' into design

Conflicts:
	app/helpers/application_helper.rb
	app/views/users/show.html.slim
	db/schema.rb
	spec/helpers/application_helper_spec.rb