Feature: Users should be able to create and add custom drugs at for interview-intake

[motey]

At the moment, when a drug is not included in the Arzneimittelindex, there is no way of including it in an interview as intake. This can happen if a drug is very new on the market.
We need endpoints and a UI for this.

Basic Requierments:

• Add a drug as intake that is created on the fly

Nice2Have/Advanced features

• Make this custom drug available for all users in the search (Must be visibile as "non-arzneimittelindex/custom drug")
• Create a merge feature if the drug comes with a newer version of the Arzneimittelindex

[motey]

Backend implementation happens in this branch https://github.com/DZD-eV-Diabetes-Research/DZDMedLog/tree/rewrite/feat-customdrugs

[motey]

@JTaeger

New endpoints:
POST - /drug/user-custom Create a custom drug
GET - /drug/user-custom/my List the current user custom drugs
GET - /drug/user-custom/{custom_drug_id} Get a custiom drug by its id

Changes in datamodel:
Intake has now a new field custom_drug_id which can be alternatively filled out with a custom drug id, when there is not PZN available.

(As of 29.05 this is all still only in the feat-customdrugs branch