Clean up old news

[Chris Warburton]

mu/mu4e is getting quite slow at indexing. We want to keep our emails,
but news can pile up, especially from feeds like the BBC and HackerNews.

We should (already?) remove files from ~/Mail/feeds/foo which are
e.g. older than a certain date, or whatever. We aren't currently
removing them from the mu index database.

The 'mu remove' command will hopefully do this for us:

  https://man.voidlinux.eu/man1/mu-remove.1

It seems to require a filename, which doesn't have to exist. We could,
for example, do a search for all BBC news items, loop through each one
in bash and check if the file exists; if not, do mu remove to drop it
from the index. And so on for other high-volume feeds.