Split "modules/context.go" to separate files (#24569)

The "modules/context.go" is too large to maintain.

This PR splits it to separate files, eg: context_request.go,
context_response.go, context_serve.go

This PR will help:

1. The future refactoring for Gitea's web context (eg: simplify the context)
2. Introduce proper "range request" support
3. Introduce context function

This PR only moves code, doesn't change any logic.

routers/api/v1/repo/repo.go

@@ -178,7 +178,7 @@ func Search(ctx *context.APIContext) {
 		if len(sortOrder) == 0 {
 			sortOrder = "asc"
 		}
-		if searchModeMap, ok := context.SearchOrderByMap[sortOrder]; ok {
+		if searchModeMap, ok := repo_model.SearchOrderByMap[sortOrder]; ok {
 			if orderBy, ok := searchModeMap[sortMode]; ok {
 				opts.OrderBy = orderBy
 			} else {