Added ENV variables for Navbar logo and to show/hide Contribute link

As per Jon Lemmon's suggestion, to achieve an easy way to update the "Loomio" logo with a custom logo on a self hosted instance, I added a method to application_helper.rb and modified the HAML as well. 

I also added a method that uses similar ENV variable setting to either "hide" or default "show" the Contribute heart link in the navbar!

Now, to specify your own custom logo / hide the Contribute link set the following two values in .env file

NAVBAR_LOGO_PATH="mailpile-logo-90-35.png"
NAVBAR_CONTRIBUTE="hide"