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