Add schema migration for new tables

[jschaul]

• schema migration for remote users in local conversations; and users -> remote conversations
• partially implements adding a remote user to a local conversation: cassandra queries are made use of (data is actually inserted), however checks are missing and that endpoint is still hidden from actual usage by using a /i/ prefix.
• augments OtherMember to also give back a qualified user Id; this means that querying for a conversation (get /conversations/:cnv) supports showing remote users. For this, golden tests had to be adjusted, which accounts for some of the noise of this PR.
• removing a remote member is implemented at the cql query level but never actually used nor tested. That is a futurework.
• There may be some minor conflicts with the work in Port instances to schemas library #1482 (we needed some instances to make servant work); use the instances of Port instances to schemas library #1482 when resolving conflicts.
• Adds scaffolding for internal endpoints in galley with servant.
• Adds lots of FUTUREWORKs.

See the comment in https://github.com/wireapp/wire-server/pull/1485/files#diff-259be1b8909f30421ab02e60765d6e90ae68426b95c3aa8303c57945dc84dd37R467-R481 for some next steps that can be implemented once this PR is merged.

[pcapriotti]

I have taken the liberty to merge develop, and then remove JSON instances that were covered by schemas.

Looks good to me. I've added a few minor inline comments.

[pcapriotti]

This is not normally necessary, because servant-swagger has a more specific instance for Verb ... NoContent. Here we need it because NoContent is inside UVerb, but I'm not sure this is the best way.

I think this is fine for now, but for the purposes of submitting a PR, I think the proper way to support NoContent in UVerb is to have some more specific instances for ToSwagger that do not necessarily require ToSchema. I have done something similar for HasServer and HasClient in my servant PR.