Implement models required by Manifest API

[ekigamba]

The following models will be required by the Manifest API:

• Manifest

The table holding the manifests will have the following columns:

Column name	Column Type	Nullable	Indexes	Misc
id	int	no	PK	auto_incremented
json	JSONB	no
app_version	String	no	b_tree
created_at	long	no
updated_at	long	no

The manifest will be stored inside the json field