Add new managed resources implementation

A new ManagedStorage interface allows blobs to be stored in a mongo database. The blobs are initially namespaced to an environment but apis will be added to the interface to allow data to be stored globally or against a given user.

Data is de-duped when Put - if there is already a blob with the same checksums (md5/sha256) it is not stored twice; a reference to the original data is used.
```go
storagePath := resourcePath
if user != "" {
    storagePath = path.Join("users", user, storagePath)
}
if environUUID != "" {
    storagePath = path.Join("environs", environUUID, storagePath)
}
```
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