Record and display catalog as charts available per namespace

[absoludity]

The previous task (#1496) enabled the asset-sync service to sync charts across all namespaces. This task continues that work to enable the dashboard to display the charts available per namespace. See design doc for more info and discussion.

Follows on from #1496, parent task #1256.

• Ensure that per-namespace app repositories are sync'd using their defined repo credentials (sync job needs these passed from kubeapps copy)
• asset-sync: Record the namespace in the assetsvc (db - mongo / postgresql) when sync'ing chart data (so a chart can exist in multiple namespaces, or _all)
• assetsvc: update requests to the chart-svc for charts to expect both the users credentials as well as the namespace being requested, returning charts for the namespace if the user has access.
• UI: Update dashboard to request and display charts per namespace when viewing the catalog (behind the feature flag)