Use image service for background images of checkboxes and selects

Removes use of o-ft-icons code point in checkboxes and data uri in selects.

Open issues:
* Tint doesn't seem to work for icons in the image service
* Image service sometimes returns a 403
* Products can't serve their own image resources
Remove dependancy on o-ft-icons
Use o-assets and local files for checkbox and select background images
Merge branch 'master' into checkbox-select-bg-images
Merge branch 'master' of https://github.com/Financial-Times/o-ft-forms into checkbox-select-bg-images

Conflicts:
	bower.json
	main.scss
	src/scss/radio-checkbox.scss
Remove redundant function
Use o-ft-icons instead of own svgs
Merge pull request #54 from Financial-Times/checkbox-select-bg-images

Checkbox select bg images