Remove avatar related commands from protocol and tables from model

Avatars are managed by the API.
There is no more need to have this in the protocol except for python client.

Why now? We recently removed the `url` column from the database and replaced it with a `filename` column. The main reason behind this change is that we want to decouple the filename from the url (we want to be able to move file around if we need to). Then the prefix to the content server should be configuration only (which it is now in the API).

For now the `url` column is kept for backward compatibility by adding the prefix hardcoded, but we want to remove this. Therefore we either put the url prefix as configuration in the server or we just remove the protocol and the tables from the server which reduces complextiy which is unneeded.

This will break the avatars in the python client, but due to url encoding issues we solved now in the api they are partially broken already.
Alternative for the python client: Use the avatar endpoints available since Q1 2018.