Bento customization small improvements

Related to #777 (closed) Related to #772 (closed) Related to #773 (closed)

• Alphabetizes Predefined Bento in selects (and fixes an issue where the filterific sorting was case-sensitive and always put eBooks after all the PBs that started with capital letters)
• Disambiguates predefined bentos by using “Display Name (code)” most places
• Fixes the issue Mike noticed where the configured bento display name placeholder no longer updated when you changed predefined bentos
• Puts all references to “Active?” behind a feature flag and, for now, changes “Shown by Default?” to “Visible?” on configured bento pages. (Following through on my promise to hide one of two checkboxes if we went live without user customization)
• Adds links from the configured bento show page to predefined bento and bento config
• Makes it possible for inst users to see the show and index pages for predefined bentos (once we turn on their access to any bento stuff), if they navigate from a configured bento
• Conditionally hides edit/delete buttons on the predefined bento show/index pages depending on user’s permissions
• Hides ConfiguredBento.slug everywhere except the show page, and computes it based on display name, or predefined bento code, since GSS thought the whole concept of slug was too confusing for inst users. Now the slug will be the predefined bento code, unless you’ve overwritten the display name; then it’s a slugified version of the display name. If it’s not unique, it gets suffixed -1, -2, etc