Add a setting to label ads as such

Problem/Motivation

In a lot of regions, it is required to label ads as actually being advertisements rather than an actual part of the webpage by, e.g., showing a small “AD” text in the corner or similar things. For that reason a simple text setting (that we could put into the settings tab of the module which is already there) that will be shown in a lower corner of the ad would suffice.

Steps to reproduce

See that no such setting exists currently, and one would have to use a template or JS to add an “AD” label.

Proposed resolution

Implement the proposed setting and its functionality.

Remaining tasks

User interface changes

API changes

Data model changes