Introduce named routes

I have replaced the hard coded routes with Rails' named routes.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Make test code pithier

By using further Capybara primitives the test code could be refactored.
A utilities files helps by determining the proper page title. All files
inside spec/support are automatically included by RSpec.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Generate repository controller

For the next steps I want to make the set of used repositories visible
in the web interface. Hence, I have created a repositories controller,
added the appropriate route and updated the links in the header layout.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>