Basic set notation exercise

Added a basic set notation exercise covering unions, intersections and
differences.
Changed from 2 space indents to 4

Whoops. Bad Sublimetext - no donut!
Missing div tag in the vars section

Missed it when reformatting the code. Isn't caching swell.
Removed data-ensure clauses

Fixes performance issues.
Allow empty answers to be correct when they pass

handleSubmit() didn't entertain the idea that empty answers can still
be correct. I've tried all the other exercises with 'custom' answers
and can't find anything that breaks.

I could just add an 'empty set' checkbox ala
absolute_value_equations.html or button that adds a empty sign to the
field but it seems unintuitive.
Minor hint changes
Merge pull request #39358 from davemajor/set-notation

Add exercise: Basic Set Notation