Validate migration files (#18203)

JSON Schema validation for data used by Gitea during migrations

Discussion at https://forum.forgefriends.org/t/common-json-schema-for-repository-information/563

Co-authored-by: Loïc Dachary <loic@dachary.org>

modules/migration/reaction.go

@@ -6,7 +6,7 @@ package migration
 
 // Reaction represents a reaction to an issue/pr/comment.
 type Reaction struct {
-	UserID   int64  `yaml:"user_id"`
-	UserName string `yaml:"user_name"`
-	Content  string
+	UserID   int64  `yaml:"user_id" json:"user_id"`
+	UserName string `yaml:"user_name" json:"user_name"`
+	Content  string `json:"content"`
 }