Fixed conflicting svg ids
Fixed conflicting svg ids

no issue

- enables `prefixIds` svgo option to guarantee all `id` attributes are unique across svgs
- disabled `cleanupIds` option so that it doesn't interfere with id uniqueness
- fixes instagram icon background not showing

TryGhost/Ghost-Admin,kevinansfield/Ghost-Admin,kevinansfield/Ghost-Admin,TryGhost/Ghost-Admin