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)

services/repository/contributors_graph.go

@@ -143,7 +143,6 @@ func getExtendedCommitStats(repo *git.Repository, revision string /*, limit int
 		PipelineFunc: func(ctx context.Context, cancel context.CancelFunc) error {
 			_ = stdoutWriter.Close()
 			scanner := bufio.NewScanner(stdoutReader)
-			scanner.Split(bufio.ScanLines)
 
 			for scanner.Scan() {
 				line := strings.TrimSpace(scanner.Text())
@@ -180,7 +179,6 @@ func getExtendedCommitStats(repo *git.Repository, revision string /*, limit int
 					}
 				}
 				commitStats.Total = commitStats.Additions + commitStats.Deletions
-				scanner.Scan()
 				scanner.Text() // empty line at the end
 
 				res := &ExtendedCommitStats{