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="https://github.com/go-gitea/gitea/assets/32161460/2603504f-aee7-4929-a8c4-fb3412a7a0f6">

After:
<img width="1296" alt="image"
src="https://github.com/go-gitea/gitea/assets/32161460/58c03721-729f-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>

templates/repo/navbar.tmpl

@@ -5,4 +5,7 @@
 	<a class="{{if .PageIsContributors}}active {{end}}item" href="{{.RepoLink}}/activity/contributors">
 		{{ctx.Locale.Tr "repo.activity.navbar.contributors"}}
 	</a>
+	<a class="{{if .PageIsCodeFrequency}}active{{end}} item" href="{{.RepoLink}}/activity/code-frequency">
+		{{ctx.Locale.Tr "repo.activity.navbar.code_frequency"}}
+	</a>
 </div>