Move API pagination into a common partial

[jhawthorn]

Previously, pagination was handled individually by each index partial, which led to some inconsistencies.

This PR adds a spree/api/shared/pagination.json.jbuilder partial which is passed a paginated collection and renders the pagination attributes.

This doesn't change any existing attributes, but some endpoints will now have per_page and total_count, which were missing previously.

[tvdeyen]

Looks good.

Thinking of adding a change log entry just in case someone has overwritten these views exactly for that use case (total_count vs. count) and would not need this anymore.

[mamhoff]

Nice work. We will already have a changelog entry for all the views being converted from RABL to JBuilder, so I think that should cover it.

[kennyadsl]

👍