Provide endpoints to submit tasks to re-generate previews

The scaling factor can be given in the config file. When this changes,
images can be regenerated via POSTing to certain endpoints. It is
possible to regenerate just one attachment preview or all within a
collective.

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

@@ -18,6 +18,12 @@ object MakePreviewArgs {
 
   val taskName = Ident.unsafe("make-preview")
 
+  def replace(attach: Ident): MakePreviewArgs =
+    MakePreviewArgs(attach, StoreMode.Replace)
+
+  def whenMissing(attach: Ident): MakePreviewArgs =
+    MakePreviewArgs(attach, StoreMode.WhenMissing)
+
   sealed trait StoreMode extends Product {
     final def name: String =
       productPrefix.toLowerCase()