Setting up discuss.bitrise.io

Basics of discuss.bitrise.io configured, anyone can now register and ask questions.
Still tweaking some details and adding a couple of initial categories.

Initial categories prepared with at least one example post.

Enabled GitHub login & added the “Mark as solved” and “Feature voting” plugins

Added the http://discuss.bitrise.io/ link to https://www.bitrise.io/contact, to the “?” (help) dropdown menu and to the footer of bitrise.io

SSO configured - login and registration can now be done with a Bitrise account