Stop creating duo_code_review_bot account when not necessary

What does this MR do and why?

This MR refactors the use of Users::Internal.duo_code_review_bot so it does not create a new user when organization_users_internal feature flag is enabled.

There are several instances in the codebase where we call Users::Internal.duo_code_review_bot only to fetch the username associated with that account. This is necessary mostly for the frontend to work.

Since the username is already hardcoded, we could do better by fetching it from a constant instead of fetching a whole User record from database. This will not only save a query but also avoid creating a group-level account when not necessary just because we want to expose the bot username.

One particular case we want to avoid is when a user access the homepage and app/controllers/concerns/homepage_data.rb was calling the bot method to get its username, always creating the SA.

References

Related to #565290

How to set up and validate locally

I'm relying on test suite to validate these changes and based on my smoke tests, everything should be fine.

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