Shanbady/copy edits for milestone demo

[shanbady]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/4643

Description (What does it do?)

Copy changes for the demo:

1. Homepage Search: Update button text to Browse by Topic (with no "s")
2. Homepage Featured Courses: Updates button text to 'Professional & Executive Learning'
3. Homepage topics section: Update heading text to Browse by Topic (with no "s")
4. Homepage Testimonials section: Update subheading text to Transforming education through the eyes of our learners
5. Topic Listing page heading: "Topics" -> Browse by Topic
6. Remove banner description text Topic Listing page
7. Department Listing page heading: "Departments" -> Browse by Academic Department
8. Search page: Changed text in search box to What do you want to learn?
9. Change the "Sign Up/Log In" button in header to just "Log In"

reference the ticket for screenshot/discussion

[gumaerc]

👍