First version of new ui based on tailwind

This drops fomantic-ui as css toolkit and introduces tailwindcss. With
tailwind there are no predefined components, but it's very easy to
create those. So customizing the look&feel is much simpler, most of
the time no additional css is needed.

This requires a complete rewrite of the markup + styles. Luckily all
logic can be kept as is. The now old ui is not removed, it is still
available by using a request header `Docspell-Ui` with a value of `1`
for the old ui and `2` for the new ui.

Another addition is "dev mode", where docspell serves assets with a
no-cache header, to disable browser caching. This makes developing a
lot easier.

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

@@ -40,6 +40,7 @@ case class Duration(nanos: Long) {
 }
 
 object Duration {
+  val zero = Duration(0L)
 
   def apply(d: SDur): Duration =
     Duration(d.toNanos)

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

@@ -0,0 +1,49 @@
+package docspell.common
+
+import cats.implicits._
+
+sealed trait EnvMode { self: Product =>
+
+  val name: String =
+    productPrefix.toLowerCase
+
+  def isDev: Boolean
+  def isProd: Boolean
+}
+
+object EnvMode {
+  private val sysProp = "docspell.env"
+  private val envName = "DOCSPELL_ENV"
+
+  case object Dev extends EnvMode {
+    val isDev  = true
+    val isProd = false
+  }
+  case object Prod extends EnvMode {
+    val isDev  = false
+    val isProd = true
+  }
+
+  def dev: EnvMode  = Dev
+  def prod: EnvMode = Prod
+
+  def fromString(s: String): Either[String, EnvMode] =
+    s.toLowerCase match {
+      case s if s.startsWith("dev")  => Right(Dev)
+      case s if s.startsWith("prod") => Right(Prod)
+      case _                         => Left(s"Invalid env mode: $s")
+    }
+
+  def read: Either[String, Option[EnvMode]] = {
+    def normalize(str: String): Option[String] =
+      Option(str).map(_.trim).filter(_.nonEmpty)
+
+    normalize(System.getProperty(sysProp))
+      .orElse(normalize(System.getenv(envName)))
+      .traverse(fromString)
+  }
+
+  lazy val current: EnvMode =
+    read.toOption.flatten.getOrElse(prod)
+
+}