Move admin users list to its own controller

[AntonKhorev]

Previous attempt: #4969

What I'm trying to achieve:

• free users_path helper for POST requests that create users, as explained in Move users list to administrator module #4969
• make app/controllers/users_controller.rb smaller
• remove special :index User ability available only to admins; normally :index is available together with :show; after this we can remove all :index abilities similar to Remove :edit abilities implied by :update abilities #5164 and Remove :new abilities implied by :create abilities #5185
• remove if request.post? and use two different actions for showing/updating the list
• fix: redirect here probably wasn't entirely correct because there was no params[:page] after implementing before/after pagination

[github-actions]

	1 Warning
⚠️	Number of updated lines of code is too large to be in one PR. Perhaps it should be separated into two or more?

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[tomhughes]

Looks good to me, thanks.