[Rails] Show total file storage for each community in the super admin dashboards

[rudokemper]

For a Terrastories server super admin, it can be helpful to know how much file storage is being taken up by each community. (For example, to understand the breakdown of storage costs per each community.)

Let's make this possible by adding a new card to the Community view, to the right of the existing four, with this content: {filesize} GB (summing up the total number of gigabytes, with two decimal places:

The figure should sum up the total filesize count for all of the media attachments stored for that community (e.g. photos, audio, video).

Acceptance criteria:

• As a logged in super admin, I can navigate to a community, and I see the total number of GB of their file storage.