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/msmtprc

@@ -0,0 +1,24 @@
+defaults
+
+port 587
+tls on
+tls_trust_file /etc/ssl/certs/ca-certificates.crt
+
+# Aprila mail
+account aprila
+from vincent@aprila.no
+host smtp.office365.com
+auth on
+user vincent@aprila.no
+passwordeval pass show aprila/office365-mail
+
+# Runbox mail
+account runbox
+from mail@tazj.in
+host mail.runbox.com
+auth on
+user mail@tazj.in
+passwordeval pass show general/runbox-tazjin
+
+# Use Runbox as default
+account default : runbox