Add "after" slots

[pehbehbeh]

In addition to this, content can be inserted between the main positions via the following extra spots: :before_page_title, :before_actions, :before_filters, :before_metrics and :before_main.

https://hexdocs.pm/backpex/0.5.1/Backpex.LiveResource.html#c:render_resource_slot/3

Currently it is not possible to insert custom content on all actions after the page title because :before_actions is only shown on index (not on edit and show).