Adds Ddr::Admin.curators_group setting.

Defaults to DDR_CURATORS_GROUP env var or the DDR Curators policy group. In the development Docker Compose environment, it is set to "registered" so that any/every user is a curator.

Since we are intending to use this group for a wide array of authorizations (see https://duldev.atlassian.net/wiki/spaces/DDRKB/pages/30285470/Repository+Groups#Groups-for-Authorizing-Other-Repository-Abilities), we have injected a #curator? method into Ddr::Auth::AuthContext, which returns a boolean based on the user's membership status in the curators group.

We have replace the usage of the collection creators group from ddr-core, and so have replaced the Ddr::Auth::CollectionAbilityDefinitions with Ddr::Admin::CollectionAbilityDefinitions.

The curators group is also used to provide access to the queues and Solr web UIs from the "user menu".

In an unrelated change we have deprecated the "saved searches" menu item since that feature has been removed from Blacklight in version 7 (see DDR-2521).