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/activity.tmpl

@@ -8,6 +8,7 @@
 		<div class="flex-container-main">
 			{{if .PageIsPulse}}{{template "repo/pulse" .}}{{end}}
 			{{if .PageIsContributors}}{{template "repo/contributors" .}}{{end}}
+			{{if .PageIsCodeFrequency}}{{template "repo/code_frequency" .}}{{end}}
 		</div>
 	</div>
 </div>

templates/repo/code_frequency.tmpl

@@ -0,0 +1,9 @@
+{{if .Permission.CanRead $.UnitTypeCode}}
+	<div id="repo-code-frequency-chart"
+		data-locale-loading-title="{{ctx.Locale.Tr "graphs.component_loading" (ctx.Locale.Tr "graphs.code_frequency.what")}}"
+		data-locale-loading-title-failed="{{ctx.Locale.Tr "graphs.component_loading_failed" (ctx.Locale.Tr "graphs.code_frequency.what")}}"
+		data-locale-loading-info="{{ctx.Locale.Tr "graphs.component_loading_info"}}"
+		data-locale-component-failed-to-load="{{ctx.Locale.Tr "graphs.component_failed_to_load"}}"
+	>
+	</div>
+{{end}}

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>