Use a minimum age of items to remove

In order to keep deleted items for a while, the periodic task can now
use a duration to only remove items with a certain age. This can be
used to ensure that a deleted item stays at least X days before it
will be removed from the database.

Refs: #347

modules/common/src/main/scala/docspell/common/EmptyTrashArgs.scala

@@ -18,11 +18,12 @@ import io.circe.generic.semiauto._
   * items. These are items with state `ItemState.Deleted`.
   */
 case class EmptyTrashArgs(
-    collective: Ident
+    collective: Ident,
+    minAge: Duration
 ) {
 
   def makeSubject: String =
-    "Empty trash"
+    s"Empty Trash: Remove older than ${minAge.toJava}"
 
 }