Blob storage, history feature and performance improvements

Today's update (0b662f0) brings with it the following changes:

Performance improvements

Object content is now stored in an external service called BlobMachine, making it possible to scale to multiple web servers.
All objects are stored named by the digest checksum of the content, improving caching because identical files only need to be read once. This feature also improves the transactional behavior during object modification.

Whenever objects are modified the old version of content and metadata is stored, allowing quick restoration of content back to a previous version. Use the history button in the toolbar to access this feature. The history is kept for 6 weeks.
It's also possible to restore deleted containers and objects, but because of a bug they are restored to the root instead of their proper location. When restoring content on an existing object or container the parent is maintained. This issue will be fixed in a future update.

Cache files are now properly named after the digest checksum, which allows them to be created only once, becaues they never change without the name also changing.
The cache directory is regularly cleaned of old files, recovering disk space more efficiently.
The timeout in thumbnail(r) was increased to account for the increased time it takes to fetch videos from the blob service.

The thumbnail request handler was unable to generate thumbnails for SVG images. Changed to return the SVG image directly when asked for a thumbnail of it. It should be able to scale to the needed size because it's a vector image.