Support for following a thread that the signed in user didn't create

[christianselig]

I think at some point it'd be cool to have a feature where if a user finds an interesting thread (perhaps asking a question relevant to them) they could subscribe to/follow that thread and get notifications about new (likely only top-level) comments on the thread.

For instance, UserA posts a thread in their city's subreddit wondering when the new Batman movie will be showing. UserB (signed in and using Apollo and also from this city) sees this thread and there's 0 comments, but is curious to see what responses will be, so requests Apollo to notify them.

Examples of user requesting this:

• https://www.reddit.com/r/apolloapp/comments/thymn8/feature_request_follow_a_post_for_notifications/

• https://www.reddit.com/r/apolloapp/comments/tm8yg1/ability_to_track_a_post_for_comments/

[christianselig]

Other things discussed:

• Following an individual comment in a thread too instead of just the post itself
• When following a post option to only get notifications for OP's comments (think an AMA where you don't want every reply only replies by the person doing the AMA)

Should discuss:

• How to handle enormous threads (like an AskReddit one)