Display ZFS & Services memory utilisation in Dashboard

[boswelja]

TrueNAS web shows a chart with a breakdown of memory usage by ZFS cache, services, and free memory. Example:

This data is provided to us via the API, but currently we only display used and free memory. It'd be cool if we showed what was used by what.

[boswelja]

We now display (what seems like) caches in the dashboard! There is no label for it though