Move POST /{username}/action/{action} to simply POST /{username} (#18045) (#18046)

Backport #18045

The current code unfortunately requires that `action` be a reserved
repository name as it prevents posts to change the settings for
action repositories. However, we can simply change action handler
to work on POST /{username} instead.

Fix #18037

Signed-off-by: Andrew Thornton <art27@cantab.net>

routers/web/web.go

@@ -461,9 +461,7 @@ func RegisterRoutes(m *web.Route) {
 		m.Get("/attachments/{uuid}", repo.GetAttachment)
 	}, ignSignIn)
 
-	m.Group("/{username}", func() {
-		m.Post("/action/{action}", user.Action)
-	}, reqSignIn)
+	m.Post("/{username}", reqSignIn, user.Action)
 
 	if !setting.IsProd() {
 		m.Get("/template/*", dev.TemplatePreview)