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>

modules/structs/repo.go

@@ -154,7 +154,7 @@ type CreateRepoOption struct {
 	// Whether the repository is template
 	Template bool `json:"template"`
 	// Gitignores to use
-	Gitignores string `json:"gitignores"`
+	Gitignores string `json:"gitignores" binding:"MaxSize(1024)"`
 	// License to use
 	License string `json:"license" binding:"MaxSize(100)"`
 	// Readme of the repository to create