Instance/Static runtime plug

This allows to set-up an arbitrary directory which overrides most of the
static files: index.html static/ emoji/ packs/ sounds/ images/ instance/
favicon.png.

If the files are not present in the directory, the bundled ones in
priv/static will be used.

test/plugs/instance_static_test.exs

@@ -0,0 +1,43 @@
+defmodule Pleroma.Web.RuntimeStaticPlugTest do
+  use Pleroma.Web.ConnCase
+
+  @dir "test/tmp/instance_static"
+
+  setup do
+    static_dir = Pleroma.Config.get([:instance, :static_dir])
+    Pleroma.Config.put([:instance, :static_dir], @dir)
+    File.mkdir_p!(@dir)
+
+    on_exit(fn ->
+      Pleroma.Config.put([:instance, :static_dir], static_dir)
+      File.rm_rf(@dir)
+    end)
+  end
+
+  test "overrides index" do
+    bundled_index = get(build_conn(), "/")
+    assert html_response(bundled_index, 200) == File.read!("priv/static/index.html")
+
+    File.write!(@dir <> "/index.html", "hello world")
+
+    index = get(build_conn(), "/")
+    assert html_response(index, 200) == "hello world"
+  end
+
+  test "overrides any file in static/static" do
+    bundled_index = get(build_conn(), "/static/terms-of-service.html")
+
+    assert html_response(bundled_index, 200) ==
+             File.read!("priv/static/static/terms-of-service.html")
+
+    File.mkdir!(@dir <> "/static")
+    File.write!(@dir <> "/static/terms-of-service.html", "plz be kind")
+
+    index = get(build_conn(), "/static/terms-of-service.html")
+    assert html_response(index, 200) == "plz be kind"
+
+    File.write!(@dir <> "/static/kaniini.html", "<h1>rabbit hugs as a service</h1>")
+    index = get(build_conn(), "/static/kaniini.html")
+    assert html_response(index, 200) == "<h1>rabbit hugs as a service</h1>"
+  end
+end