Refactoring FeatureSpecLoginHelper class.

Mostly by extracting expected base functionality to new
`FeatureSpecHelperBase` class, and moving it and the new class from
`spec/support` to `spec/support/feature_spec`. This of course required
touching every feature spec file that explicitly included the login
helper.

RSpec: 419 examples, 0 failures
RuboCop: 115 files inspected, no offenses detected