Change "Teacher" to "Instructor"

# Story
```
As an educator,
I want the term used to refer to me to be "Instructor",
So that it reflects best across the full spectrum of education levels.
```
# Acceptance Criteria:
- There are no references to "Teacher".
- All references to the user-educator use the term "Instructor".
    - This includes occurrences in the user interface, the product documentation, and implementation.