[IMPROVEMENT] Honor Accounts_ShowFormLogin setting

[Caiocbeleza]

@RocketChat/ReactNative

Closes #1123

[diegolmello]

You should create your own server and test if it behaves correctly.
We have two similar login views: LoginView and LoginSignupView.
If there're no login services, the user is redirected to LoginView.
Also, it seems Accounts_ShowFormLogin hides register as well.

[ankar84]

We will use only KeyCloak as only one identify provider for our Rocket.Chat, and we disabled all other things, such as registration form, logon form, password reminder and other.
In browser or desktop application logon page shows only on KeyCloak button - it's good.
But RN mobile app shows Login with e-mail button and Create an account link - it's bad.
So we totally need this feature!
Can you finish it so we can get it merged?

[djorkaeffalexandre]

Hey guys.
It'll be solved by #1727.
Closing it now.
Thanks.