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)

routers/web/web.go

@@ -1448,6 +1448,10 @@ func registerRoutes(m *web.Route) {
 				m.Get("", repo.Contributors)
 				m.Get("/data", repo.ContributorsData)
 			})
+			m.Group("/code-frequency", func() {
+				m.Get("", repo.CodeFrequency)
+				m.Get("/data", repo.CodeFrequencyData)
+			})
 		}, context.RepoRef(), repo.MustBeNotEmpty, context.RequireRepoReaderOr(unit.TypePullRequests, unit.TypeIssues, unit.TypeReleases))
 
 		m.Group("/activity_author_data", func() {