Decouple the different contexts from each other (#24786)

Replace #16455

Close #21803

Mixing different Gitea contexts together causes some problems:

1. Unable to respond proper content when error occurs, eg: Web should
respond HTML while API should respond JSON
2. Unclear dependency, eg: it's unclear when Context is used in
APIContext, which fields should be initialized, which methods are
necessary.


To make things clear, this PR introduces a Base context, it only
provides basic Req/Resp/Data features.

This PR mainly moves code. There are still many legacy problems and
TODOs in code, leave unrelated changes to future PRs.

services/forms/package_form.go

@@ -25,6 +25,6 @@ type PackageCleanupRuleForm struct {
 }
 
 func (f *PackageCleanupRuleForm) Validate(req *http.Request, errs binding.Errors) binding.Errors {
-	ctx := context.GetContext(req)
+	ctx := context.GetValidateContext(req)
 	return middleware.Validate(errs, ctx.Data, f, ctx.Locale)
 }