Implement code frequency graph (#29191)

### Overview
This is the implementation of Code Frequency page. This feature was
mentioned on these issues: #18262, #7392.

It adds another tab to Activity page called Code Frequency. Code
Frequency tab shows additions and deletions over time since the
repository existed.

Before:
<img width="1296" alt="image"
src="2603504f-aee7-4929-a8c4-fb3412a7a0f6">

After:
<img width="1296" alt="image"
src="58c03721-729f-4536-a663-9f337f240963">

---

#### Features
- See additions deletions over time since repository existed
- Click on "Additions" or "Deletions" legend to show only one type of
contribution
- Use the same cache from Contributors page so that the loading of data
will be fast once it is cached by visiting either one of the pages

---------

Co-authored-by: Giteabot <teabot@gitea.io>
(cherry picked from commit 875f5ea6d83c8371f309df99654ca3556623004c)

web_src/js/utils/color.js

@@ -19,3 +19,17 @@ function getLuminance(r, g, b) {
 export function useLightTextOnBackground(r, g, b) {
   return getLuminance(r, g, b) < 0.453;
 }
+
+function resolveColors(obj) {
+  const styles = window.getComputedStyle(document.documentElement);
+  const getColor = (name) => styles.getPropertyValue(name).trim();
+  return Object.fromEntries(Object.entries(obj).map(([key, value]) => [key, getColor(value)]));
+}
+
+export const chartJsColors = resolveColors({
+  text: '--color-text',
+  border: '--color-secondary-alpha-60',
+  commits: '--color-primary-alpha-60',
+  additions: '--color-green',
+  deletions: '--color-red',
+});