Fix missing abilities for Browse pages and Viewers

[chrisrlc]

• https://culibrary.atlassian.net/browse/LP-706: Add published Browse groups to public pages
  • Also gave curators ability to update browse groups, which appeared to be missing when comparing our Spotlight::Ability to https://github.com/projectblacklight/spotlight/blob/v3.5.0.2/app/models/spotlight/ability.rb
• https://culibrary.atlassian.net/browse/LP-707: Allow users with Viewer role to view unpublished exhibits

Not sure why these were missing - maybe they were missed during a previous Spotlight upgrade?