Add feature flag "conferenceCalling"

[smatting]

https://wearezeta.atlassian.net/browse/SQSERVICES-681

This PR also:

• refactor some of the feature endpoint implementation
• adds to test that all simple flags should emit a feature-config.update event

Checklist

• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• If end-points have been added or changed: the endpoint / config-flag checklist (see Wire-employee only backend wiki page) has been followed.
• If a schema migration has been added, I ran make git-add-cassandra-schema to update the cassandra schema documentation.
• Section Unreleased of CHANGELOG.md contains the following bits of information:
  • A line with the title and number of the PR in one or more suitable sub-sections.
  • If /a: measures to be taken by instance operators.
  • If /a: list of cassandra migrations.
  • If public end-points have been changed or added: does nginz need upgrade?
  • If internal end-points have been added or changed: which services have to be deployed in a specific order?

[jschaul]

Could you update the docs/config-options.md and explain what it means for this to be enabled or disabled, and what the default is?

[smatting]

Could you update the docs/config-options.md and explain what it means for this to be enabled or disabled, and what the default is?

Done

[fisx]

LGTM!

No point in changing this now, but I would have preferred 2-3 smaller PRs (at least the unrelated tests should have been trivial to do separately).

[fisx]

this seems unrelated?