Statistics : number private links, database and cache size

[ArthurHoaro]

Discussed in #106:

• Add counters for number of public and private links to the tools page.
• Add counters for datastore and cache directories to the tools page.

Maybe highlight cache size if it's anormaly big.

[nodiscc]

My vote goes to only separate number of public/private links counters. db and cache size can be checked with a simple file manager/sftp/ftp client, and they should not be that large

For me:

$ du -hc data/datastore.php cache/ tmp/
292K    data/datastore.php
18M cache/
120K    tmp/
19M total

Other numbers are welcome.

Related to #185