Add gde_name and total_impact as computed fields

`gde_name` is now retrieved from `Account` based on `gplus_id` (fixes https://github.com/maiera/gde-app/issues/98)

`total_impact` is computed for the AR from all `impact` values in metadata records, which should make it easy to display this value next to +1's/comments/reshares in reports. (see https://github.com/maiera/gde-app/issues/93)

Both fields will be silently ignored if supplied in insert/update calls, so existing API calls that include the `gde_name` from the frontend won't cause an issue.