Remove Turnip BDD gem

The main reason for removing Turnip is the "extra layer" of complexity when it comes to debugging of failing tests: one always has to open the feature file first, then find the failing step. With plain Capybara feature specs, all the debugging needs are in the same file, which is a lot easier.

Wit this decision, we lose some flexibility of improving existing specs by improving steps, and we lose some central ways to ensure uniformity throughout different scenarios (as in Capybara feature specs, we typically copy&paste a lot of code, while in Turnip, we simply copy&paste the steps' names).

As we don't really do acceptance testing but only feature testing anyways, I think I have learned quite a bit about the pros and cons of BDD using Turnip (or Cucumber). But for pragmatical reasons, we abandon it and stick with plain Capybara feature specs. :+1: