Use similar directory hierarchy for thumbnails as for attachments

At the moment the <CONF_HOME>/attachments and <CONF_HOME>/thumbnails directories have two different hierarchy structures, would be good to keep these similar.

If there are enough pages creating thumbnails in Confluence, then the 32000 file limit of ext2/3 will be reached, as mentioned by G2G JP Support in a comment below.

Workarounds

1. Use a different filesystem with a higher file limit. ext4 has 64000 by default, and can be increased higher using "dir_nlink"
2. Periodically clear out old thumbnail files so that the limit is not reached - since they are dynamically generated, the files will be repopulated as needed