Move membership icons

Moving the membership icons to a new sprite as it added 30kb to global.css

Icons should now only be loaded on identity pages.

cc @kaelig 
Agreed the conditional would be as such within the template:
```
@if(projectName === 'identity') {
    <link src="membership-icons.css" rel"stylesheet"/>"
}
```
This way you are ensuring it is only loaded on identity pages.