Provide method for users to join back session

[CJianzhi]

lgtm, works as intended, code looks clean as well

referring to your PR description, I'm not sure about the 2nd case, and my questions/concerns are:

1. how would the user be able to navigate back to the collab page after leaving?
2. should the code editor state be saved instead of being deleted? in line with our nice-to-have feature of saving code editor info after 5mins (iirc)

maybe something nice to have would be to have highlight tracking as well? so lets say a user is referring to one chunk of code when speaking with the other, and would like to bring up that chunk for discussion, they can highlight and both be able to see it. but if you do decide to add this, I think it can go in another PR

Originally posted by @hyc17003 in #22 (review)