Add storage and bandwidth usage to instance dashboard

Is your feature request related to a problem? Please describe

The instance dashboard should display some analytics.

Describe the solution you'd like

The instance dashboard should display:

• Number of uploaded bytes per month as a graph

SELECT sum(file_size), DATE_FORMAT(`updated_at`,'%Y-%m') AS month FROM `cp_media` GROUP BY month ORDER BY month;

• Number of total uploaded bytes

SELECT sum(file_size) FROM `cp_media`;

• Number of downloaded bytes per months as a graph

SELECT sum(`bandwidth`), DATE_FORMAT(`updated_at`,'%Y-%m') AS month FROM `cp_analytics_podcasts` GROUP BY month ORDER BY month;