Admin log-in using devise

[veelenga]

Create sign-in page, login/logout actions:

https://github.com/plataformatec/devise

For now, only admin should be able to login, create/update/delete(CUD) resources and see CUD buttons/links. But probably in future we will let other users to be able to login, so we can start creating role model.

Also we have to deny access on controller side, so the user will not bypass. Each of those might help:

https://github.com/CanCanCommunity/cancancan
https://github.com/elabs/pundit
https://github.com/nathanl/authority