Introduce the `ActionText::RichTextEditorTest` to exercise Action Text's
integration with Active Storage. First, assert that Action Text renders
an Active Storage blob preview of the image within the `<trix-editor>`
element prior to persisting the rich-text record. Next, assert that
Active Storage renders the uploaded file within the rendered rich-text
content.

In addition to expanding System Test coverage, this commit also alters
the existing `ActionText::SystemTestHelperTest` coverage to utilize
Capybara's built-in [:element][] selector as an alternative to CSS. The
`:element` selector supports arbitrary keyword arguments as HTML
attribute filters. For example, `input: true` as a filter translates to
asserting the presence of any `[input]` attribute. Similarly, it enables
assertions with Ruby strings instead of interpolating Ruby strings
**into** CSS selectors.

[:element]: https://rubydoc.info/github/teamcapybara/capybara/master/Capybara/Selector