mail: Update to new neomutt structure

Updated notmuch to only be available for full-text search. Removed
complete afew configuration.

Added msmtp as mail sending agent.

Added structured neomutt configuration with custom bindings, built for
my personal gmail account, without any plaintext passwords revealed,
etc.

This is a reasonably well working mail setup,
which should provide a stable starting point for further refinements.

Additionally, added some functionality to `mail-check` script:
Allowed user to choose sync target for each run, by
passing in mbsync target (group/channel/..) as the argument.
Also, allows setting password file to use for script through env var,
and made imapfilter location less hard-coded to my system.

mail/.config/msmtp/config

@@ -0,0 +1,18 @@
+# Set default values for all following accounts.
+defaults
+auth           on
+tls            on
+tls_trust_file /etc/ssl/certs/ca-certificates.crt
+# logfile        ~/.msmtp.log
+
+# Gmail
+account        personal-gmail
+host           smtp.gmail.com
+port           587
+from           marty.oehme@gmail.com
+user           marty.oehme@gmail.com
+# password       plain-text-password-goes-here
+passwordeval  "gpg --quiet --for-your-eyes-only --no-tty --decrypt $XDG_DATA_HOME/pass/misc/aerc-gmail-app-password.gpg"
+
+# Set a default account
+account default : personal-gmail