Add a "do not boost recency" option for some accounts
Bot accounts (e.g. nowplaying bots) can be useful for populating a list, but I don't want them continuously popping existing entries up -- this floods out accounts that may only post music a few times a day.
To address this, add an option to mark an account as subject to the following behavior:
- If a marked account is the first account to mention some URL, add it to all chronologically-ordered lists in the correct position.
- If a marked account mentions an existing URL, do not use that marked account's status to modify the existing URL's position in chronologically-ordered lists.
This might address #17.