fix inconsistent capitalization of sign-up/sign-in links

* original PR #131 rebased on latest upstream master
* made consistent capitalization of all Sign-Up and Sign-in links
* added links:sign_up: translation placeholders for all languages (need translations)
* added general:or: transations for all languages
* all oauth providers are listed in Home#index 'Contribute' section
* added ApplicationHelper#list_friendly_text method for the previous feature