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;
```
issue