[us7] export pdf backend support

[joon0429]

Estimated Effort: 8 hours

Dependencies: None

Initial Assignment: Jerry Chen

Milestone: Sprint 2

Acceptance Criteria:

• backend properly triggers some export or print functionality
• routes are tested to prompt default browser print functionality
• may need to subtask for exporting single-post threads v. multi-post threads
• does not necessarily need to have robust formatting if the thread is >1 page