mopidy: restart service on config changes

[jwillikers]

Description

The Mopidy service is not restarted when the config changes.
This PR adds X-Restart-Triggers to the unit to ensure that the service is restarted on config file changes.

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

• Test cases updated/added. See example.

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Maintainer CC

@foo-dogsquared

[rycee]

Thanks! I extended the test a bit and merged to master now 🙂