using slice_id instead of url to do caching

[yxjames]

I use slice_id as the cache key. When slices are cached from dashboard I add manually add slice_id into form_data since sometimes form_data['slice_id'] is empty.

[coveralls]

Coverage increased (+0.02%) to 82.127% when pulling 0ff5ec7 on joshwalters:dashboard_cache into 087c47a on airbnb:master.

[yxjames]

If the slice is accessed from dashboard, form_data['slice_id'] is empty until the user overwrite that slice. I don't know if this is designed for some purpose. Any idea?

[coveralls]

Coverage increased (+0.02%) to 82.127% when pulling f7e974e on joshwalters:dashboard_cache into 087c47a on airbnb:master.

[mistercrunch]

I don't think this works where filter widget is used.