feat(mail): Import email configuration into Nix

This moves my email configuration into NixOS, including the following
features:

1. Replaced the NixOS-builtin offlineimap user service with a custom
   one that runs notmuch-indexing as part of the systemd unit instead
   of a postsynchook, which is significantly more reliable.
2. Adds configuration for notmuch and its tagging.
3. Adds configuration for OfflineIMAP & MSMTP.

Relevant emacs configuration has also been added to my emacs.d
repository.

dotfiles/notmuch-config

@@ -0,0 +1,21 @@
+# .notmuch-config - Configuration file for the notmuch mail system
+#
+# For more information about notmuch, see https://notmuchmail.org
+
+[database]
+path=/home/vincent/mail
+
+[user]
+name=Vincent Ambo
+primary_email=mail@tazj.in
+other_email=vincent@aprila.no;tazjin@gmail.com;
+
+[new]
+tags=unread;inbox;
+ignore=
+
+[search]
+exclude_tags=deleted;spam;
+
+[maildir]
+synchronize_flags=true