gnu: mumi: Update to 0.0.0-12.bb2fe92.

* gnu/packages/mail.scm (mumi): Update to 0.0.0-12.bb2fe92.
[inputs]: Add guile-gcrypt, guile-redis, guile-webutils, mailutils.
* gnu/services/web.scm (<mumi-configuration>): New record type.
(mumi-configuration, mumi-configuration?, mumi-configuration-mumi,
mumi-configuration-mailer?, mumi-configuration-sender,
mumi-configuration-smtp): New procedures.
(mumi-shepherd-services): Accept configuration; adjust start commands; add
Shepherd service for mumi mailer.
(mumi-service-type): Pass default configuration.
* doc/guix.texi (Web Services): Update documentation.