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.

models/repo.go

@@ -35,9 +35,6 @@ import (
 	"xorm.io/builder"
 )
 
-// ItemsPerPage maximum items per page in forks, watchers and stars of a repo
-var ItemsPerPage = 40
-
 // Init initialize model
 func Init(ctx context.Context) error {
 	if err := unit.LoadUnitConfig(); err != nil {