Show the directory size of server

[jojomatik]

Is your feature request related to a problem? Please describe.
It is currently not possible to see the directory size from the GUI.

Describe the solution you'd like
It is possible to see the server size from the GUI.

Describe alternatives you've considered
None.

Additional context
Some plugins produce a lot of small files (e.g. dynmap). Therefore, it might take very long to calculate the exact size of all subdirectories.