Rename dspam "Spam" folder "Junk"

[taoeffect]

I don't know why sovereign doesn't already do this (given that they linked to this blog post that uses it), but it seems like installing and setting up dovecot-antispam might be a good idea.

[taoeffect]

Make sure the folder is called "Junk", not "Spam", and lives in the root of each mailbox.

[taoeffect]

so @PiPeep observed that this gets installed already (in dspam.yml). Still need to make sure the folder is called "Junk" (for client compat).

[taoeffect]

nm, looks like it already supports and does it.