fix inconsistent capitalization of signup/sign-in links

* made consistent capitalization of all Sign-Up and Sign-in links
* added home:contribute:or: transations for all languages
* added links:sign_up: translation placeholders for all languages (need translations)
* all oauth providers are listed in Home#index 'Contribute' section
* added HomeHelper#list_friendly_text method for the previous feature
* add cucumber tests for signup and sign-in links
* added features/signup_signin.feature
* added features/step_definitions/home_steps.rb
* factored out commonality from some existing steps (ie. "I go to ...") (same as a22263c in #126)