Add support for more generic notification

This is a start to have different kinds of notifications. It is
possible to be notified via e-mail, matrix or gotify. It also extends
the current "periodic query" for due items by allowing notification
over different channels. A "generic periodic query" variant is added
as well.

modules/restserver/src/main/resources/logback.xml

@@ -9,7 +9,7 @@
 
   <logger name="docspell" level="debug" />
   <logger name="emil" level="debug"/>
-
+  <logger name="org.http4s.server.message-failures" level="debug"/>
   <root level="INFO">
     <appender-ref ref="STDOUT" />
   </root>