Change relocation behaviour to not create a new organisation and instead use the existing one.

### Problem Statement

The relocation process requires users to create a new organisation and go to `/relocation` from that organisation, for example `ogrimmar.sentry.io/relocation`, and start the relocation process.

The current relocation process will create a new organisaton `ogrimmar-xx` instead of using the existing organisation. The expectation in some cases is that the existing organisation is used.

Additonally, if the existing organisation is already being used this creates conflicts as the relocated project and members are added to a new organisation and options need to be migrated between them.

### Solution Brainstorm

Have the relocation tool use the existing organisation.

This can cause conflicts and a decision needs to be made regarding what to do when:
- organisation settings are different, overwrite or keep the existing settings
- projects with the same slug already exist
- teams with the same slug already exist
- users already exist in Sentry

### Product Area

Org Relocation

The issue names the /relocation entry point; start by tracing the existing relocation flow and how it selects or creates an organisation. Before implementation, resolve behavior for conflicting settings, project and team slugs, and existing users; done means the agreed flow reuses the existing organisation without requiring migration.

Written by the indexing model from the issue text.