SQSERVICES-547 add feature config for guest links

[battermann]

Implementation for https://wearezeta.atlassian.net/browse/SQSERVICES-1045 and https://wearezeta.atlassian.net/browse/SQSERVICES-1046 for Epic

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]

last ci failure looks like a flake:

brig                             >     sftDiscoveryLoop
brig                             >       when service can be discovered:                                 OK
brig                             >       when service can be discovered and the URLs change:             FAIL
brig                             >         test/unit/Test/Brig/Calling.hs:169:
brig                             >         servers should get overwritten
brig                             >         expected: Discovered (SFTServers (SrvEntry {srvPriority = 0, srvWeight = 0, srvTarget = SrvTarget {srvTargetDomain = "sft4.foo.example.com.", srvTargetPort = 443}} :| [SrvEntry {srvPriority = 0, srvWeight = 0, srvTarget = SrvTarget {srvTargetDomain = "sft5.foo.example.com.", srvTargetPort = 443}}]))
brig                             >          but got: Discovered (SFTServers (SrvEntry {srvPriority = 0, srvWeight = 0, srvTarget = SrvTarget {srvTargetDomain = "sft1.foo.example.com.", srvTargetPort = 443}} :| [SrvEntry {srvPriority = 0, srvWeight = 0, srvTarget = SrvTarget {srvTargetDomain = "sft2.foo.example.com.", srvTargetPort = 443}},SrvEntry {srvPriority = 0, srvWeight = 0, srvTarget = SrvTarget {srvTargetDomain = "sft3.foo.example.com.", srvTargetPort = 443}}]))

but i also forgot to push something important. it's now there.

[fisx]

today's not a good day for concourse....