Standardize background-image styles

* Dropped -moz-linear-gradient when present
* Only embed linear-gradient – if we are embedding just one thing,
  it is only fair that the modern browsers get most benefit
* Added svg versions to places where missing
* Filed a bug 60948 for images missing svg version

Change-Id: I3177a161a1961f03e86f990d611e57e66dc6040e
Merge "Standardize background-image styles"