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.

project/Dependencies.scala

@@ -17,6 +17,7 @@ object Dependencies {
   val Fs2Version = "2.3.0"
   val H2Version = "1.4.200"
   val Http4sVersion = "0.21.1"
+  val Icu4jVersion = "66.1"
   val KindProjectorVersion = "0.10.3"
   val Log4sVersion = "1.8.2"
   val LogbackVersion = "1.2.3"
@@ -218,4 +219,7 @@ object Dependencies {
     "org.webjars" % "viewerjs" % ViewerJSVersion
   )
 
+  val icu4j = Seq(
+    "com.ibm.icu" % "icu4j" % Icu4jVersion
+  )
 }