Create endpoint to get v5 user id by user handle

[maxceem]

We already added this endpoint GET /v5/taas-teams/users/{handle}:

• this endpoint is needed to get v5 userId by handle
• in case a user with such a handle doesn't exist in v5, then this endpoint creates it automatically and returns userId

This was a quick fix via commit 2b7a591, so we have to improve the solution:

• Update Swagger
• Rename SkillController / SkillService to something more appropriate like UbahnController / UbahnService, or UserController/UserService
• Add Joi validation
• decide on permissions to access this endpoint, should it be more strict?

[maxceem]

Probably have to remove this endpoint along with #112. We should decide if we can implement everything without this.

[maxceem]

This was reverted as we don't need it anymore 2b6ccdb. We are going to migrate users from V3 to V5, so we wouldn't need to ensure user exists anymore.