Remove association between ChatName and Integration - Step 1

What does this MR do and why?

Per issue, this MR does the following:

• Removes the integration_id foreign key from chat_names
• Drops the existing UNIQUE index on (integration_id, team_id, chat_id)
• Replaces the UNIQUE index on (user_id, integration_id) index with a non-unique index just on user_id, by adding the non-unique index, and then dropping the other one
• Updates the validations in ChatName:
  • Removes the uniqueness validation on :user_id that is scoped to integration_id
  • Updates the uniqueness validation on :chat_id to be scoped only to :team_id

How to set up and validate locally

1. Checkout the current branch
2. Run rails db:migrate
3. Validate that everything works

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #384963 (closed)