Include a solution box for the domain of the expression
Remove redundant variable
Split final hint
Merge pull request #43433 from petercollingridge/domain-q

Students must state the domain of simplified expression where necessary.

fix #43504; fix #43362;

Test plan: try a few example exercises (note that 1 of the 3 types don't require a domain to be specified) http://localhost:8000/exercises/simplifying_rational_expressions_3.html

Auditors: eater, alpert, cbhl