Add Empty Spam command
Closes #5829
Not tested yet, just copy and pasted what I found of the "Empty Trash" command.
Seems legacy/core/src/test/java/com/fsck/k9/controller/PendingCommandSerializerTest.java can be left as-is?
Thanks for the pull request! At a first glance it looks good to me :+1:
Sorry, for not providing any feedback earlier. You have probably noticed that we're very busy with getting ready for the first release of Thunderbird for Android. We don't want to include new unplanned features before the release. However, we'll get back to this after the initial release of Thunderbird for Android.
I applied the necessary fixes myself and created #8750.
Sorry for the hold up, and thank you!