Add sponsoring mentioning icon and title to disciplines about lunes-cms

[timobrembeck]

Add two new fields to discipline response with icon and title

Since we already have a sponsor's model, I suggest to use a foreign key instead.

Add new sponsors endpoint where we can get a sponsor information for specific discipline

The sponsors API already exists: https://lunes.tuerantuer.org/api/docs/#/sponsors/sponsors_list

So it's sufficient to:

1. Either add the sponsor id to the response of the discipline endpoint
2. Or add a filter to the existing sponsor endpoint to allow the filtering by discipline

[roman-digitalfabrik]

We decided to keep the disciplines sponsors out of the modules.
All sponsor mentioning happens only in the new Sponsor page in the App (which is already editable in the CMS). If neccessary we could add information there.

[MizukiTemma]

Closed as not needed anymore