Opt-in to react notifications on old posts
imported from https://todo.sr.ht/~luma_inhibitor/cables/51
Conditions:
- A user places a reaction on a post where the reactor isn't the author
- The author has this user config item enabled
Matching case logic:
- Evaluate when the OP was made, and if it is older then n days, send a DM to the author notifying them of the react, when it was made, the contents of the OP, and a message link to it
- If a server or user config item is overriden, use that value as the required post age instead of the default of 3 days