new: language auto-detect - /api/guess-language, refresh button, guess on paste

main.go

@@ -327,6 +327,7 @@ func (a *apiServer) routes() http.Handler {
 		r.Delete("/pastes/{id}", a.handleDeletePaste)
 		r.Delete("/pastes/{id}/redeem", a.handleRedeemDeletion)
 		r.Get("/public", a.handleListPublic)
+		r.Post("/guess-language", a.handleGuessLang)
 		r.Post("/pastes/can", a.handleCreateCan)
 		r.Get("/cans/{id}", a.handleGetCan)
 		r.Get("/cans/{id}/items/{item}", a.handleCanItem)