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.

modules/web/middleware/flash.go

@@ -7,11 +7,13 @@ import (
 	"fmt"
 	"html/template"
 	"net/url"
+
+	"code.gitea.io/gitea/modules/reqctx"
 )
 
 // Flash represents a one time data transfer between two requests.
 type Flash struct {
-	DataStore ContextDataStore
+	DataStore reqctx.RequestDataStore
 	url.Values
 	ErrorMsg, WarningMsg, InfoMsg, SuccessMsg string
 }