Add rubocop-capybara to require using path matching helper

[mitchellhenke]

🛠 Summary of changes

We've had some flakier feature tests recently following #11605, and some of them have been alleviated by doing replacements like:

-    expect(current_path).to eq sign_up_completed_path
+    expect(page).to have_current_path sign_up_completed_path

The second line uses the Capybara has_current_path? helper, which includes a timeout to wait for page loads rather than asserting the current_path in that moment in time. This will hopefully make some of the longer or more complicated feature test steps more reliable. There may still be places where we're missing an assertion and should add it before trying to take other actions like clicking on a button.

This PR adds rubocop-capybara and enables Capybara/CurrentPathExpectation to require usage of the have_current_path helper.

[aduth]

LGTM