Small updates to student assignment invitation flow

[femmebot]

Problem The language on the classroom invitation page is confusing. The primary action seems to encourage students to skip the step. [See notes from teacher feedback]

Overall, the language and presentation of the invitation flow seemed to borrow heavily from GitHub Classroom which is designed for the teacher, not the student.

Proposed updates include:

• Clarified language for the roster so students skip the step only if they don't find their name/identifier
• Removed the header for the classroom since it doesn't have any utility for students (students shouldn't have access to the teacher's classroom itself) and it takes up so much real estate
• Other smaller copy edits and stylistic updates to make the content readable and easier to understand.

Before

After

[femmebot]

Closing this in favor of https://github.com/education/classroom/pull/2492