Modify luminance calculation and extract related functions into single files (#24586)

Close #24508

Main changes:
As discussed in the issue

1. Change luminance calculation function to use [Relative
Luminance](https://www.w3.org/WAI/GL/wiki/Relative_luminance)
2. Move the luminance related functions into color.go/color.js
3. Add tests for both the files (Not sure if test cases are too many
now)

Before (tests included by `UseLightTextOnBackground` are labels started
with `##`):
https://try.gitea.io/HesterG/testrepo/labels

After:
<img width="1307" alt="Screen Shot 2023-05-08 at 13 37 55"
src="https://user-images.githubusercontent.com/17645053/236742562-fdfc3a4d-2fab-466b-9613-96f2bf96b4bc.png">
<img width="1289" alt="Screen Shot 2023-05-08 at 13 38 06"
src="https://user-images.githubusercontent.com/17645053/236742570-022db68e-cec0-43bb-888a-fc54f5332cc3.png">
<img width="1299" alt="Screen Shot 2023-05-08 at 13 38 20"
src="https://user-images.githubusercontent.com/17645053/236742572-9af1de45-fb7f-460b-828d-ba25fae20f51.png">

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Giteabot <teabot@gitea.io>

modules/util/color_test.go

@@ -0,0 +1,65 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+package util
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_HexToRBGColor(t *testing.T) {
+	cases := []struct {
+		colorString string
+		expectedR   float64
+		expectedG   float64
+		expectedB   float64
+	}{
+		{"2b8685", 43, 134, 133},
+		{"1e1", 17, 238, 17},
+		{"#1e1", 17, 238, 17},
+		{"1e16", 17, 238, 17},
+		{"3bb6b3", 59, 182, 179},
+		{"#3bb6b399", 59, 182, 179},
+		{"#0", 0, 0, 0},
+		{"#00000", 0, 0, 0},
+		{"#1234567", 0, 0, 0},
+	}
+	for n, c := range cases {
+		r, g, b := HexToRBGColor(c.colorString)
+		assert.Equal(t, c.expectedR, r, "case %d: error R should match: expected %f, but get %f", n, c.expectedR, r)
+		assert.Equal(t, c.expectedG, g, "case %d: error G should match: expected %f, but get %f", n, c.expectedG, g)
+		assert.Equal(t, c.expectedB, b, "case %d: error B should match: expected %f, but get %f", n, c.expectedB, b)
+	}
+}
+
+func Test_UseLightTextOnBackground(t *testing.T) {
+	cases := []struct {
+		r        float64
+		g        float64
+		b        float64
+		expected bool
+	}{
+		{215, 58, 74, true},
+		{0, 117, 202, true},
+		{207, 211, 215, false},
+		{162, 238, 239, false},
+		{112, 87, 255, true},
+		{0, 134, 114, true},
+		{228, 230, 105, false},
+		{216, 118, 227, true},
+		{255, 255, 255, false},
+		{43, 134, 133, true},
+		{43, 135, 134, true},
+		{44, 135, 134, true},
+		{59, 182, 179, true},
+		{124, 114, 104, true},
+		{126, 113, 108, true},
+		{129, 112, 109, true},
+		{128, 112, 112, true},
+	}
+	for n, c := range cases {
+		result := UseLightTextOnBackground(c.r, c.g, c.b)
+		assert.Equal(t, c.expected, result, "case %d: error should match", n)
+	}
+}