fix(repo): limit gitignore template selections (#39027) (#39040)

Backport #39027 by @bircni

Bound gitignore template selections at both web and API request
boundaries before repository initialization.

Co-authored-by: bircni <bircni@icloud.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/forms/repo_form.go

@@ -28,7 +28,7 @@ type CreateRepoForm struct {
 	Description   string `binding:"MaxSize(2048)"`
 	DefaultBranch string `binding:"GitRefName;MaxSize(100)"`
 	AutoInit      bool
-	Gitignores    string
+	Gitignores    string `binding:"MaxSize(1024)"`
 	IssueLabels   string `binding:"MaxSize(255)"`
 	License       string `binding:"MaxSize(100)"`
 	Readme        string `binding:"MaxSize(255)"`