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_form.go

@@ -0,0 +1,72 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package context
+
+import (
+	"strconv"
+	"strings"
+
+	"code.gitea.io/gitea/modules/util"
+)
+
+// FormString returns the first value matching the provided key in the form as a string
+func (ctx *Context) FormString(key string) string {
+	return ctx.Req.FormValue(key)
+}
+
+// FormStrings returns a string slice for the provided key from the form
+func (ctx *Context) FormStrings(key string) []string {
+	if ctx.Req.Form == nil {
+		if err := ctx.Req.ParseMultipartForm(32 << 20); err != nil {
+			return nil
+		}
+	}
+	if v, ok := ctx.Req.Form[key]; ok {
+		return v
+	}
+	return nil
+}
+
+// FormTrim returns the first value for the provided key in the form as a space trimmed string
+func (ctx *Context) FormTrim(key string) string {
+	return strings.TrimSpace(ctx.Req.FormValue(key))
+}
+
+// FormInt returns the first value for the provided key in the form as an int
+func (ctx *Context) FormInt(key string) int {
+	v, _ := strconv.Atoi(ctx.Req.FormValue(key))
+	return v
+}
+
+// FormInt64 returns the first value for the provided key in the form as an int64
+func (ctx *Context) FormInt64(key string) int64 {
+	v, _ := strconv.ParseInt(ctx.Req.FormValue(key), 10, 64)
+	return v
+}
+
+// FormBool returns true if the value for the provided key in the form is "1", "true" or "on"
+func (ctx *Context) FormBool(key string) bool {
+	s := ctx.Req.FormValue(key)
+	v, _ := strconv.ParseBool(s)
+	v = v || strings.EqualFold(s, "on")
+	return v
+}
+
+// FormOptionalBool returns an OptionalBoolTrue or OptionalBoolFalse if the value
+// for the provided key exists in the form else it returns OptionalBoolNone
+func (ctx *Context) FormOptionalBool(key string) util.OptionalBool {
+	value := ctx.Req.FormValue(key)
+	if len(value) == 0 {
+		return util.OptionalBoolNone
+	}
+	s := ctx.Req.FormValue(key)
+	v, _ := strconv.ParseBool(s)
+	v = v || strings.EqualFold(s, "on")
+	return util.OptionalBoolOf(v)
+}
+
+func (ctx *Context) SetFormString(key, value string) {
+	_ = ctx.Req.FormValue(key) // force parse form
+	ctx.Req.Form.Set(key, value)
+}