Fix converted filename

Mark it by default with a string from the config file.

Issue: 397

modules/convert/src/main/scala/docspell/convert/ConvertConfig.scala

@@ -6,6 +6,7 @@ import docspell.convert.flexmark.MarkdownConfig
 
 case class ConvertConfig(
     chunkSize: Int,
+    convertedFilenamePart: String,
     maxImageSize: Int,
     markdown: MarkdownConfig,
     wkhtmlpdf: WkHtmlPdfConfig,

modules/convert/src/test/scala/docspell/convert/ConversionTest.scala

@@ -23,6 +23,7 @@ object ConversionTest extends SimpleTestSuite with FileChecks {
 
   val convertConfig = ConvertConfig(
     8192,
+    "converted",
     3000 * 3000,
     MarkdownConfig("body { padding: 2em 5em; }"),
     WkHtmlPdfConfig(