Create admin facing audit log for login as user feature

[kceb]

https://www.pivotaltracker.com/story/show/177614355

Context
I would like to be able to set custom top level meta attributes. Current jsonapi-utils does not support it out of the box. jsonapi-resources has some support for it w/custom operation processors, however, based on experimentation, these custom operation processors are not yet compatible with jsonapi-utils. Rather than doing extensive changes to this forked version of jsonapi utils to support custom operation processors, I figure it would be easier to allow custom meta declaration.

Inspiration: tiagopog#91

Why?
We have an admin portal page for admin events. The admin events live on the SP events DB (separate from SP core).

We need pagination on this admin index page. ActiveAdmin supports two types of pagination

1. With total count -- slow for large tables b/c of COUNT(*)
2. Without total count -- faster for large tables b/c lack of COUNT(*)

Option 2 makes most sense for this table, however the way ActiveAdmin determines this is by seeing if there is a result on the next page. I could make two requests to determine this or I could provide a custom meta attribute saying has_next_page: true. I tested the former locally with ~21 million records and it takes ~20 seconds to render a page. The latter has the opportunity to be more performant b/c it can be done in one request.

Meta would be declared in admin events index action

[kceb]

Talked with Noah, we are going an alternate route