Move away from the "andForget" style of function (#3043)

As suggested by @crspeller here:
https://github.com/mattermost/platform/pull/3022#issuecomment-220004020

api/oauth.go

@@ -636,5 +636,5 @@ func CompleteSwitchWithOAuth(c *Context, w http.ResponseWriter, r *http.Request,
 		return
 	}
 
-	sendSignInChangeEmailAndForget(c, user.Email, c.GetSiteURL(), strings.Title(service)+" SSO")
+	go sendSignInChangeEmail(c, user.Email, c.GetSiteURL(), strings.Title(service)+" SSO")
 }