Refactor User#follow into a service

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Why are we doing this work

As a follow-up for !97349 (comment 1104027190) and !97349 (comment 1104027193), we should refactor User#follow into a service. There are multiple benefits for this:

1. We have more flexibility with error handling, as we no longer have to rely on ActiveModel#errors to hold error messages.
2. We can associate an HTTP status code with each error
3. We have better separation of concerns as we User shouldn't be concerned with creating UserFollowUser records

Implementation plan

1. Create app/services/users/follow_service.rb
2. Move the logic from User#follow into this service
3. Delete User#follow
4. Replace usages of User#follow with Users::FollowService