SQSERVICES-547 post/get conversation/:cnv/code returns error if feature disabled

[battermann]

https://wearezeta.atlassian.net/browse/SQSERVICES-1057

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 HTTP endpoint paths have been added or renamed, the endpoint / config-flag checklist (see Wire-employee only backend wiki page) has been followed.
• If a cassandra schema migration has been added, I ran make git-add-cassandra-schema to update the cassandra schema documentation.
• changelog.d contains the following bits of information (details):
  • A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.
  • If new config options introduced: added usage description under docs/reference/config-options.md
  • If new config options introduced: recommended measures to be taken by on-premise instance operators.
  • If a cassandra schema migration is backwards incompatible (see also these docs), measures to be taken by on-premise instance operators are explained.
  • If a data migration (not schema migration) introduced: measures to be taken by on-premise instance operators.
  • If public end-points have been changed or added: does nginz need un upgrade?
  • If internal end-points have been added or changed: which services have to be deployed in a specific order?

[fisx]

Both this and #1976 look great, but please add integration tests.

• wire-server/services/galley/test/integration/API.hs

  Line 216 in 402bdf7

  test s "join code-access conversation" postJoinCodeConvOk,

  - add code (i think?)
• not so sure where get is called?
• I also just realized that if a conversation is created, there is the option of adding a guest link right away. That should also trigger the same error (can be done here or in a separate PR; I'll add it to the ticket.)

[fisx]

I'm thinking what would be the right level of aggregation here? Maybe keep this but start with "enforce disabled conv links."?

[fisx]

LGTM! (I probably would have tried to eliminate code duplication in the two tests, but this also works.)