* store the S3 uri in the 'store_fname' (s3://bucket/key)
* the read-only mode is now built-in, as we store the bucket name, if a
instance is started with a different bucket or another filestore
method, it will continue to read the previous attachments on their
stored bucket, but new attachments will be stored on the new one
* remove config in ir.config_parameter, it makes all the stuff more
complex and we don't use them (config file would be more interesting)
* automatically migrate the attachments on loading of the server, so
if an ir.attachment has been created during the module
upgrade/initialization before attachment_s3 is loaded, it will be sent
to S3 as soon as it's loaded
When we return a bin_size (such as 28672), the web client does not
accept it as as bin size and try to use it as value for the b64 src of
the image. Though, returning the image as b64 in all cases does seem to
be supported.
Guewen Baconnier