[API] Change Resource Serializer to Pull `user` field values from Currently Authed User

[BethanyG]

The current resource POST (create) api will take any int value POSTed for the user field. This means, effectively, that one authorized user can "impersonate" another when it comes to creating a resource in the DB.

We don't want this to happen. Instead, the user field should be pulled from and populated by the 'username' field from the JWT token.

See this Stack Overflow Post for information on how this can be accomplished.

see DRF: Generic Views perform_create() under Save and deletion hooks for additional context.

[BethanyG]

I am going to assign this to myself, since I've included this in my current PR for bug #43 .