More dynamic and better way for hooks

[javrasya]

Descriptions:

Hooking to a specific event currently doesn't fulfill the main idea of django-river. Create new hooking or updating an existing one requires a code change and a redeploy.

Acceptance Criteria:

• The current way of registering a hook via a django-river python API should be replaced by a dynamic way like creating them on the DB and have django-river see them. This way changes would be applied immediately on the hooks as well.
• Callback functions should also be created in the DB.
• Functions shouldn't be loaded every time it is needed. There should be something like an internal version kept that changes on every update on the function.