Move context from modules to services (#29440)

Since `modules/context` has to depend on `models` and many other
packages, it should be moved from `modules/context` to
`services/context` according to design principles. There is no logic
code change on this PR, only move packages.

- Move `code.gitea.io/gitea/modules/context` to
`code.gitea.io/gitea/services/context`
- Move `code.gitea.io/gitea/modules/contexttest` to
`code.gitea.io/gitea/services/contexttest` because of depending on
context
- Move `code.gitea.io/gitea/modules/upload` to
`code.gitea.io/gitea/services/context/upload` because of depending on
context

services/context/context_template.go

@@ -0,0 +1,35 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package context
+
+import (
+	"context"
+	"time"
+)
+
+var _ context.Context = TemplateContext(nil)
+
+func NewTemplateContext(ctx context.Context) TemplateContext {
+	return TemplateContext{"_ctx": ctx}
+}
+
+func (c TemplateContext) parentContext() context.Context {
+	return c["_ctx"].(context.Context)
+}
+
+func (c TemplateContext) Deadline() (deadline time.Time, ok bool) {
+	return c.parentContext().Deadline()
+}
+
+func (c TemplateContext) Done() <-chan struct{} {
+	return c.parentContext().Done()
+}
+
+func (c TemplateContext) Err() error {
+	return c.parentContext().Err()
+}
+
+func (c TemplateContext) Value(key any) any {
+	return c.parentContext().Value(key)
+}