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.
