Support more dashboard semantics around how filters affect slices

[mistercrunch]

Currently:

• Widgets can emit filters
• The "dashboard controller" applies this filter by refreshing slices with an extra filter parameter
• Slices (in the context of a dashboard) can be made immune to filtering by editing the json configuration behind the dashboard (no ui controls are exposed to set this up)

Needed:

• Allow specifying which slices are immune to which fields.

Currently, the json_metadata for the dashboard has a filter_immune_slices key, which receives an array of slice ids. This should still be supported.

On top of that we need a new key filter_immune_slice_fields that is an object where the key is a slice_id, and the value an array of fields that it disregards. Or something equivalent.

[vera-liu]

Sorry didn't started looking into this until today, already done in: #1179