Add RSpec feature tests for New Release page

What does this MR do?

Adds RSpec feature tests for the new New Release page developed in #214244 (closed).

Note about helpers

This MR adds a number of helpers in spec/support/helpers/features/releases_helpers.rb that are only currently used in the new test file (spec/features/projects/releases/user_creates_release_spec.rb). These methods have been implemented as shared helpers so that the tests for the Edit Release page (spec/features/projects/releases/user_views_edit_release_spec.rb) can eventually be refactored to use them. These two pages share the same Vue component, so their tests are very similar.

Note about keyboard usage

I took a somewhat unusual approach when testing this page: the form is filled out entirely with keyboard shortcuts. My intention is to test both the functionality and keyboard accessibility of this page.

Related to #214244 (closed)

Closes #235602 (closed)