Rename MagmaticAlgebras to Algebras and Algebras to AssociativeUnitalAlgebras

[nthiery]

Algebras is currently the category of associative unital
algebras. #10963 further introduces the category of algebras (not
necessarily unital nor associative) as MagmaticAlgebras. It's
been decided on sage-devel [1] that MagmaticAlgebras should eventually be
renamed as Algebras, and the current Algebras to
AssociativeUnitalAlgebras (or even just
Algebras(...).Associative().Unital()).

This is not done directly in #10963 to not add a backward incompatible
change to this already huge ticket.

[1] https://groups.google.com/d/msg/sage-devel/1lZAr60N-8w/vHr6nOWrsfcJ

CC: @sagetrac-sage-combinat @simon-king-jena

Component: categories

Issue created by migration from https://trac.sagemath.org/ticket/15043