Fixed error handling copying formula question

Fixed an error in handling copying formula question types with an error
margin expressed in percentage. When this question type is copied to
another course, the field is run through .to_f() which truncates the
percent sign, so an error margin of 10% becomes an error margin of 10.
CalculatedInteraction uses .to_f for most of its text to float
conversion, but a special case needs to be introduced for this
expression. This change intercepts a field that ends with a percent
sign (and looks like a percentage) skips the conversion. This will work
to a percision of two decimal places and up to 100%.

Test Plan:
- Create two courses, CourseA and CourseB.
- In CourseA, create a quiz which contains a formula question type.
- Set the error margin to a percentage (e.g. 1%) and add the question.
- In CourseB, copy the quiz from CourseA using “Import Content” with a
content type of “Copy a Canvas Course” and at least selecting quizzes.
- Observe in CourseB that the imported quiz will have a tolerance of 1
instead of 1%.