Refactor "Content" for file uploading (#25851)

Before: the concept "Content string" is used everywhere. It has some
problems:

1. Sometimes it means "base64 encoded content", sometimes it means "raw
binary content"
2. It doesn't work with large files, eg: uploading a 1G LFS file would
make Gitea process OOM

This PR does the refactoring: use "ContentReader" / "ContentBase64"
instead of "Content"

This PR is not breaking because the key in API JSON is still "content":
`` ContentBase64 string `json:"content"` ``

routers/web/repo/editor.go

@@ -284,10 +284,10 @@ func editFilePost(ctx *context.Context, form forms.EditRepoFileForm, isNewFile b
 		Message:      message,
 		Files: []*files_service.ChangeRepoFile{
 			{
-				Operation:    operation,
-				FromTreePath: ctx.Repo.TreePath,
-				TreePath:     form.TreePath,
-				Content:      strings.ReplaceAll(form.Content, "\r", ""),
+				Operation:     operation,
+				FromTreePath:  ctx.Repo.TreePath,
+				TreePath:      form.TreePath,
+				ContentReader: strings.NewReader(strings.ReplaceAll(form.Content, "\r", "")),
 			},
 		},
 		Signoff: form.Signoff,