Refactor request context (#32956)

Introduce RequestContext: is a short-lived context that is used to store
request-specific data.

RequestContext could be used to clean form tmp files, close context git
repo, and do some tracing in the future.

Then a lot of legacy code could be removed or improved. For example:
most `ctx.Repo.GitRepo.Close()` could be removed because the git repo
could be closed when the request is done.

services/context/context_test.go

@@ -26,6 +26,7 @@ func TestRemoveSessionCookieHeader(t *testing.T) {
 }
 
 func TestRedirectToCurrentSite(t *testing.T) {
+	setting.IsInTesting = true
 	defer test.MockVariableValue(&setting.AppURL, "http://localhost:3000/sub/")()
 	defer test.MockVariableValue(&setting.AppSubURL, "/sub")()
 	cases := []struct {
@@ -40,8 +41,7 @@ func TestRedirectToCurrentSite(t *testing.T) {
 		t.Run(c.location, func(t *testing.T) {
 			req := &http.Request{URL: &url.URL{Path: "/"}}
 			resp := httptest.NewRecorder()
-			base, baseCleanUp := NewBaseContext(resp, req)
-			defer baseCleanUp()
+			base := NewBaseContextForTest(resp, req)
 			ctx := NewWebContext(base, nil, nil)
 			ctx.RedirectToCurrentSite(c.location)
 			redirect := test.RedirectURL(resp)