refactoring navbar as component with slot for prepend component

[hudajkhan]

Closes #3167 .

What this pull request does:

• Capture the logic previously in the views/shared/_exhibit_navbar partial into a component (Spotlight::ExhibitNavbarComponent), providing a slot for content to prepend in the view. The slot will allow implementation for search tips in Stanford's Spotlight implementation.
• Replicate the view based tests in the component test, updating specific view configuration/context information.
• Add a test that checks if overriding the prepend_to_search method will display that content in the page.
• Remove the view based test from spec/views/shared.
• The _exhibit_navbar partial now renders the component. The code checks for catalog controller configuration for the Exhibit Navbar Component to display. If none is available, Spotlight::ExhibitNavbarComponent is used.
• A deprecation method has also been added in order to support the use of a component within blacklight configuration.

[dnoneill]

I think this should probably be defined in the controller somewhere. i.e. in the catalogcontroller you should define

config.index.exhibit_navbar_component = Spotlight::ExhibitNavbarComponent

I would also update the deprecation warning to point to the catalog controller where it is set.

[hudajkhan]

I think I've addresses both comments @dnoneill

[corylown]

Suggested change

	<% Spotlight.deprecator.warn("_exhibit_navbar.html.erb will be deprecated in future versions. Please use the catalog controller's exhibit_navbar_component setting.") %>
	<% Spotlight.deprecator.warn("_exhibit_navbar.html.erb will be removed; customize the exhibit navbar using components instead") %>

[dnoneill]

Sorry when I meant update the comment I mean to link or point in some way to https://github.com/projectblacklight/spotlight/pull/3171/files#diff-6052daee924920472438cf797a49bcfdafd3787945da870c8b24e9dac6bf27e4R19

[hudajkhan]

@corylown I used a period instead of a semi-colon but otherwise it is the same :)

[dnoneill]

Think this is config.index?

[hudajkhan]

yes! thanks.

[corylown]

I guess the question is, does it make sense to configure a different navbar depending on whether you're in the index or show view, or should this be configurable at the app level like the header component?

[dnoneill]

I don't think it matters, but either this or this https://github.com/projectblacklight/spotlight/pull/3171/files#diff-de18dc9392ca38402d097f5d638e554a17c8b0ba6b2e341fe46ca6188e05cf1cR9 line need to be updated to be inline with each other.