* DEV: Add a dedicated Admin::StaffController base controller
The current parent(Admin:AdminController) for all admin-related controllers
uses a filter that allows only staff(admin, moderator) users.
This refactor makes Admin::AdminController filter for only admins as the name suggests and
introduces a base controller dedicated for staff-related endpoints.
* DEV: Set staff-only controllers parent to Admin::StaffController
Refactor staff-only controllers to inherit newly introduced
Admin::StaffController abstract controller. This conveys the
purpose of the parent controller better unlike the previously used parent
controller.
It's very easy to forget to add `require 'rails_helper'` at the top of every core/plugin spec file, and omissions can cause some very confusing/sporadic errors.
By setting this flag in `.rspec`, we can remove the need for `require 'rails_helper'` entirely.
This change both speeds up specs (less strings to allocate) and helps catch
cases where methods in Discourse are mutating inputs.
Overall we will be migrating everything to use #frozen_string_literal: true
it will take a while, but this is the first and safest move in this direction
