Improve handling encodings

Html and text files are not fixed to be UTF-8. The encoding is now
detected, which may not work for all files. Default/fallback will be
utf-8.

There is still a problem with mails that contain html parts not in
utf8 encoding. The mail text is always returned as a string and the
original encoding is lost. Then the html is stored using utf-8 bytes,
but wkhtmltopdf reads it using latin1. It seems that the `--encoding`
setting doesn't override encoding provided by the document.

build.sbt

@@ -161,7 +161,8 @@ val files = project.in(file("modules/files")).
   settings(
     name := "docspell-files",
     libraryDependencies ++=
-      Dependencies.tika,
+      Dependencies.tika ++
+      Dependencies.icu4j,
     Test / sourceGenerators += Def.task {
       val base = (Test/resourceDirectory).value
       val files = (base ** (_.isFile)) pair sbt.io.Path.relativeTo(base)