Page cache not dependent on configuration

Problem/Motivation

Whenever changing settings, the new settings require a cache clear to take effect.

Steps to reproduce

1. Change settings of the module
2. Check whether settings change took effect
3. Notice there was no change
4. Rebuild caches
5. Notice there was a change

Proposed resolution

Imitate e.g. google_analytics.module to add the cacheability metadata of the module's configuration.

Remaining tasks

Perform change. Perhaps introduce a test too.

User interface changes

None.

API changes

None.

Data model changes

None.