Break out references to other objects into own tables instead of putting them as JSON

[bitfl0wer]

In tables which contain references to other Rust objects, those references should be foreign keys to a table where that object is stored in, instead of being contained in the first table as JSON.

Example:

• A table guilds exists
• Table guilds has a column owner, which represents a user
• Instead of storing the user as JSON in the guilds table, the owner column should contain a foreign key to the users table, where the user is actually stored.

Todos:

• Remove Json<T> wrapper from all types which should be broken out into own tables
• Rewrite migrations statements to contain foreign keys instead of JSON objects #41