Remove Ransack from collection model; use new component instead for table header links.

[eddierubeiz]

Ref #2450

Background Ransack did two things for us:

• It took care of some aspects of searching, filtering and sorting.
• It automatically kept track of the state of sorted tables and made it easy to create sortable column header links.

This PR separates these two tasks. The first is done in the controller; the second, inside a new component class.

More specifically:

1) Creating the list of collections to show: Searching, filtering, sorting and pagination happen entirely in the #index method, using sane and legible code.

2) Table header links:

• Default values for params are set in a new index_params method, which returns a permitted strongparams object.
• This strongparams object is used to create a factory object named :link_maker.
• :link_maker is instantiated in a before_action call, so it doesn't crowd the #index method.
• :link_maker stores and encapsulates the current state of the sorted table.
• In the template, :link_maker is then used to generate several SortedTableHeaderLinkComponent objects, one per sortable column, which are then rendered to display the actual links.

Tradeoffs Since we intend to use this component in at least three (and likely more) admin controllers, this PR aims to simplify the model, controller and view, at the expense of slightly complicating the component.

• The component, even with its internal class, is actually pretty simple considering what it does, and certainly much easier to understand than Ransack.
• Likewise, If you ignore the component and its test, and just look at the modifications this PR makes to the model, controller and view: the improvement is obvious:
  • No mention of Ransack in the model
  • For sorting and filtering, we're using tried-and-true ActiveRecord patterns instead of magical and nested param names
  • Dev works the same way as prod.
  • Search now works in dev (see note below)
  • Sorting and filtering are decoupled from generating links
  • The code in the template is short and self-explanatory

Note: The #index method was not using permitted params, so we created an index_params method to return sanitized params that #index can use. These are then handed directly to the component code, along with keys that tell the component which keys important for what it's trying to do.

Note: Ransack search is currently broken in dev on the master branch. This is one of the many reasons we want to get rid of it, but it's easy to forget about. (Try going to http://localhost:3000/admin/works in dev on master and searching for Goat. You get an ArgumentError in Admin::WorksController#index with error message Invalid search term q unless you comment out c.ignore_unknown_conditions = Rails.env.production? in ransack.rb.

What this means If you are testing this PR in dev, and note that the search is broken on the admin works page, that's not because this PR broke it. It's because it is broken in master and this PR does not modify the admin works page.

[jrochkind]

I think this is a great approach, good work!

Some comments!