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.

modules/context/context_data.go

@@ -0,0 +1,43 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package context
+
+import "code.gitea.io/gitea/modules/web/middleware"
+
+// GetData returns the data
+func (ctx *Context) GetData() middleware.ContextData {
+	return ctx.Data
+}
+
+// HasAPIError returns true if error occurs in form validation.
+func (ctx *Context) HasAPIError() bool {
+	hasErr, ok := ctx.Data["HasError"]
+	if !ok {
+		return false
+	}
+	return hasErr.(bool)
+}
+
+// GetErrMsg returns error message
+func (ctx *Context) GetErrMsg() string {
+	return ctx.Data["ErrorMsg"].(string)
+}
+
+// HasError returns true if error occurs in form validation.
+// Attention: this function changes ctx.Data and ctx.Flash
+func (ctx *Context) HasError() bool {
+	hasErr, ok := ctx.Data["HasError"]
+	if !ok {
+		return false
+	}
+	ctx.Flash.ErrorMsg = ctx.Data["ErrorMsg"].(string)
+	ctx.Data["Flash"] = ctx.Flash
+	return hasErr.(bool)
+}
+
+// HasValue returns true if value of given name exists.
+func (ctx *Context) HasValue(name string) bool {
+	_, ok := ctx.Data[name]
+	return ok
+}