mail: Switch aerc for neomutt, simplify maildir

Made mail dir synchronization simpler - only a couple of important
directories will ever get synchronized to the local file system, but
still containing basically all important incoming mails.

Removed aerc and switched it for neomutt. aerc is very nice and needs
little to no configuration but neomutt is, as of this moment, just more
stable and mature. It opens quickly, never crashes, has a nice indexing
and quick e-mail handling behavior and is incredibly customizable.
Basically, once you put in some configuration effort, it is everything
aerc isn't --- and that is good, both have their reasons for existing.
It's just that mutt currently works better for me and will thus be the
e-mail reader of choice from now on.

bootstrap/packages.txt

@@ -1,7 +1,6 @@
 # zsh plugins
 acpid
 activitywatch-bin
-aerc
 afew
 alacritty
 alias-tips-git
@@ -106,6 +105,7 @@ mpv
 mpv-mpris
 nano
 needrestart
+neomutt
 neovim
 neovim-nightly
 nerd-fonts-fira-code