Unify the way to disable certain features

Nobody has claimed this yet.

**Describe the bug**
Some features have a specific explicit config entry to en/disable them, others such as GitHub do not.

**Expected behavior**
It would be better if it were clear how to configure this. This is the case by setting an option explicitly, however removing the related config section entirely seems clean as well.

For backward compatibility, simply support both: to be enabled, a service's config section must exist and the `enabled` property must be true.

**Screenshots**
https://matrix.to/#/!TlZdPIYrhwNvXlBiEk:half-shot.uk/$WC2F16UXBuHC8H1cxw_hyHWf2lKq3a2aXPOzj3w2_XA?via=half-shot.uk&via=matrix.org&via=element.io

**Additional context**
Partially addressed in #430

Start by reviewing the configuration handling for service integrations and issue #430, which partially addresses this request. Confirm how each service is enabled today, then ensure a service is enabled only when its config section exists and its enabled property is true, while preserving both configuration forms. No files or tests are named, so identify the relevant entry points and add or update coverage for the behavior.

Written by the indexing model from the issue text.