Move admin cohorts to separate controller

What does this MR do?

1. Moves cohorts logic to its own controller.
2. Moves cohorts route from admin/users/-/cohorts to admin/cohorts so we can avoid -.

History:

1. Cohorts moved under users#index in !51707 (merged)
2. Cohorts moved under users#cohorts in !59890 (merged)

This MR reverts some of the changes made in previous merge requests for better code separation while keeping UI part the same.

Fixes #328797 (closed)

Screenshots (strongly suggested)

No visual changes

Does this MR meet the acceptance criteria?

Conformity

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• This change is backwards compatible across updates, or this does not apply.

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Security

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