digital_electricity: generalised answer checking

The tutorial levels are divided into 5 categories:

• lightTheBulb: The levels that require to light a bulb
• equation1Variable: The levels whose answers depend on 1 switch only
• equation2Variables: The levels whose answers depend on 2 switches
• equation3Variables: The levels whose answers depend on 3 switches
• others: Special case scenarios

While calculating the results in the js, we choose on which category out
of the five the current level fits in, and evaluate the answer
accordingly, removing the redundancy that existed previously.

Signed-off-by: Rudra Nil Basu <rudra.nil.basu.1996@gmail.com>