Document search filters on index page

[josegonzalez]

Refs #165

[josegonzalez]

Currently, we automatically pull search filters from the SearchBehavior's default collection. I think we can make that functionality disabled by default, with the following behavior for modifying it:

• scaffold.index_filters = null|false: default, disable filters
• scaffold.index_filters = true: auto-import default search collection
• scaffold.index_filters = 'crud': auto-import a specific search collection
• scaffold.index_filters = []: specific array of filter settings

We should also make the position of the filters changeable. They can appear:

• at the top of the page, above finder scopes but below the action header
• on the left-hand side
• on the right-hand side

For styling, I'm thinking something similar to ActiveAdmin:

Filters would need to persist the current finder scope, and clearing filters should not clear the current finder scope.

[josegonzalez]

Not sure if I want to build out this sort of rad filtering just yet or not:

[josegonzalez]

I think this can be closed as we have a good alternative now :)