[inferno-ml-server-types] Split Model and ModelVersion

[ngua]

Splits the Model type into two parts:

• Model, which now just contains metadata that should not change between versions (name, user, permissions)
• ModelVersion, which contains other version-specific metadata as well as the actual model contents

Splitting the type like this helps prevent certain undesirable scenarios. For example, all model versions previously had their own permissions. This meant that it would be possible for a user to have access to (or even own) one version of a model, but not others. That's not what we want.

Also, previously the different versions of a model were only linked together by their name field. This wasn't really structured into the database in any way. Now it is possible to get all versions of a model directly by a foreign key lookup.

I've integrated these changes on branches for the other Inferno ML projects but this will need to be merged first.

The DB tables now look like this:

,------------------------.
| Model                  |
|------------------------|
| id : serial            |
| --                     |
| * name : text          |
| * permissions : jsonb  |
| user : User (id)       |
|                        |
`------------------------'
          \|/
           o
           |           
           |           
           _
           -
,------------------------. 
| ModelVersion           | 
|------------------------| 
| id : serial            | 
| --                     | 
| * model : Model (id)   | 
| * card : jsonb         | 
| * contents : oid       | 
| * version : text       | 
|                        |
`------------------------'