Config override in settings.php for api_key fails

Problem/Motivation

When we have a config override in settings.php that overrides a fake key stored in the database like this:
$config['cocoon_media.settings']['cocoon_media']['api_key'] = 'xxxxxxxxxxxxxxxxxxx';
We get an error when trying to add cocoon media (/media/add/cocoon_media_add).

Steps to reproduce

Use the admin interface to set a dummy cocoon media api key (/admin/config/media/cocoon_media_settings).

settings.php

$config['cocoon_media.settings']['cocoon_media']['api_key'] = 'the-correct-key';

Now try to add cocoon media (/media/add/cocoon_media_add). This should fail. Then override this dummy key with a config override in settings.php. Now try again to add cocoon media. This still fails.

Proposed resolution

The CMMAddMediaForm class extends ConfigFormBase, which uses editable config that cannot be overriden with config in settings.php. There are no changes done to the configuration in this class, so it has no need for editable config. Let the class extend FormBase, and inject the ConfigFactory service to get immutable config.

Remaining tasks

See patch, it needs review.