gui, lib/api: Adds support for prefers-color-scheme (fixes #6115)

* gui, lib/api: Adds support for prefers-color-scheme on default theme (fixes #6115)

- Renames current default theme into a new "light" theme
- Modifies assets serving to allow getting assets from different themes

* lib/api: Serve assets from arbitrary theme when path starts with "theme-assets"

* lib/api: Moves constant out of function

* Loads light theme in browsers without support for prefers-color-scheme

* gui: Disables dark theme when printing

* Prevents repeated injection and adds support for older browsers

The CSS is always loaded if there is no support for `matchMedia`.

gui/light/assets/css/theme.css

@@ -0,0 +1,35 @@
+/*
+// Copyright (C) 2016 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+*/
+
+.panel-progress {
+    background: #3498db;
+}
+
+.identicon rect {
+    fill: #333;
+}
+
+.panel-warning .identicon rect {
+    fill: #fff;
+}
+
+.li-column {
+    background-color: rgb(236, 240, 241);
+    border-radius: 3px;
+}
+
+.panel-heading:hover, .panel-heading:focus {
+    text-decoration: none;
+}
+
+.fancytree-ext-filter-hide tr.fancytree-submatch span.fancytree-title,
+.fancytree-ext-filter-hide span.fancytree-node.fancytree-submatch span.fancytree-title {
+    color: black !important;
+    font-weight: lighter !important;
+}