Replacing header nav partial with header component

[hudajkhan]

Context
Relates to having tests pass with Blacklight 8.3 and using a Spotlight HeaderComponent instead of shared/_header_navbar. The Spotlight header is almost exactly the same as the default Blacklight header except it does not include the search bar (which is elsewhere on the page).
Refer to #3047 and this comment which includes a checklist of tests to fix: #3047 (comment)

For 8.3, the following tests are failing:

• rspec ./spec/views/shared/_header_navbar.html.erb_spec.rb:8 # shared/_header_navbar has nav links
• rspec ./spec/views/spotlight/pages/show.html.erb_spec.rb:46 # spotlight/pages/show when rendering with layout does not double-escape HTML entities in the HTML title
• rspec ./spec/views/spotlight/pages/show.html.erb_spec.rb:50 # spotlight/pages/show when rendering with layout includes analytics reporting

What this pull request does

• Removes the header_navbar partial.
• Creates a Spotlight HeaderComponent whose partial displays on the top bar.
• Adds the Spotlight Header Component to the configuration as the header component.
• Adds blacklight configuration information for the show html rspec to pass.

How to test
These tests should still pass in Blacklight 7.

How to test with Blacklight 8: Generate test app (.internal_test_app). Update the internal test app's Gemfile to use Blacklight 8.3. Run bundle install for the internal test app. Then run rspec ./spec/views/spotlight/pages/show.html.erb_spec.rb:46 and rspec ./spec/views/spotlight/pages/show.html.erb_spec.rb:50.

[cbeer]

I wonder if there's a backwards-compatible way to introduce this change (that won't require downstream applications to add the header_component configuration).

Maybe we could keep the header_navbar partial?

<% if blacklight_config.header_component.present? && blacklight_config.header_component != Blacklight::HeaderComponent %>
<%= render blacklight_config.header_component.new(blacklight_config: blacklight_config) %>
<% else %>
<%# TODO: deprecation warning to encourage downstream to add the config? %>
<%= render Spotlight::HeaderComponent.new(blacklight_config: blacklight_config) %>
<% end %>

[hudajkhan]

@cbeer I brought the partial back and its associated test and set up with a deprecation warning.

[jcoyne]

Suggested change

	describe 'shared/_header_navbar', type: :view do
	RSpec.describe 'shared/_header_navbar', type: :view do

[jcoyne]

Do we need to have a view test at all? View tests are deprecated in rails. Instead can you use a request test, feature test, or a component test?

[jcoyne]

Suggested change

	<% ActiveSupport::Deprecation.warn("_header_navbar.html.erb will be deprecated in future versions. Please use Blaclight configuration to specify your HeaderComponent.") %>
	<% ActiveSupport::Deprecation.warn("_header_navbar.html.erb will be deprecated in future versions. Please use Blacklight configuration to specify your HeaderComponent.") %>

[hudajkhan]

@jcoyne I addressed your suggestions. Rubocop said removing the "freeze" line at the beginning of the file was wrong (for the feature test version)

[jcoyne]

@hudajkhan needs a rebase