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>

web_src/js/components/ContextPopup.vue

@@ -26,7 +26,7 @@
 <script>
 import $ from 'jquery';
 import {SvgIcon} from '../svg.js';
-import {useLightTextOnBackground} from '../utils.js';
+import {useLightTextOnBackground, hexToRGBColor} from '../utils/color.js';
 
 const {appSubUrl, i18n} = window.config;
 
@@ -77,7 +77,8 @@ export default {
     labels() {
       return this.issue.labels.map((label) => {
         let textColor;
-        if (useLightTextOnBackground(label.color)) {
+        const [r, g, b] = hexToRGBColor(label.color);
+        if (useLightTextOnBackground(r, g, b)) {
           textColor = '#eeeeee';
         } else {
           textColor = '#111111';