Dev env: remove the necessity of having credentials locally

During !84 (merged) we stumbled upon the requirements for developing code in here as we had to add TextGrid credentials. These credentials are stored in GitLabs Secret Variables and not necessarily needed for developing code here as a bunch of test data is available and can be extended. Nevertheless the complete instance with all features require valid credentials to load the Ahikar data and the enable the proxy server for restricted images.

Do we have to have these credentials or not? If not, the tests should omit some functions. We will continue to check them in the CI pipelines anyway.

TODO: resolve the major question and – depending on the answer – check the dev documentation or rework the test scenario.

follow up to !84 (comment 269463)