[Group chat stage] Add pairwise items conversation prompts

[cjqian]

Implementation details for MVP:

• Add a parameter for number of conversation prompts (say, 5).
• Randomly select 5 pairs of items to display.
• Add some numeric indicator in the front-end to denote progress in chats (e.g. 1/5, 2/5, 3/5).
• Stage progression: For the first four, have the "I'm ready to move on" toggle say "I'm ready to discuss the next set of items" to set clear expectations. When all users in the chat have toggled, move the chat onto the next set of items. For the final stage, the text should read e.g. "I'm ready to move on."