Clear older items in _hist file after certain length of time

[julianorchard]

The only note about this would be that it should do it in a robust way: it should check how many posts from each account/subreddit/etc. is in this hist file and then only remove the links from accounts/subreddits/etc. with over a certain number of posts in the hist file.

This is because, for example, if we set the hist file to remove all but 500 lines from the hist file every time, accounts that don't post as much will have their old notifications pop up immediately, I think (depending on how each service works).