Allow admin to be split into tabs

[cogat]

Some long admin forms - especially Event admin, but any Fluent Contents admin, could do with being split into tabs for more easy navigation.

In future we want to have admin areas for content planning/briefing, and extensive help text, and tabs would help for these.

Having researched this and tried https://github.com/omji/django-tabbed-admin (which broke fluent saving and required a major template override), I think we can do this with a js-only touch, but that requires this change from Django 1.10 https://code.djangoproject.com/ticket/494 before it will work with InlineAdmins.

The idea:

• Define a tuple (in the modeladmin?) that maps Tab Titles to tab identifiers.
• Give fieldsets and InlineAdmins a tab identifier.
• Write JS to group together fieldsets and Inlines with the same tab identifier.
• Use bootstrap or JQuery-UI to make tabs.
• Maybe always put Fluent Contents, which are dynamically injected inlines, into a "Content" tab.

Note that providing a way to show which tab has errors will also be useful.