Add event templates (#4259) - #4453

Layer-scoped, event-type-specific event templates: a template is a
normal Event flagged with template: true (and inherit: true to make
it usable from layers below), excluded from all default event
queries via a default_scope, mirroring the existing
Event::QuestionTemplate pattern. Users create a new event either from
scratch or by picking an applicable template from the new-event
dropdown, and can save an existing event as a template from its show
page. Template management (create/edit/destroy) is restricted to the
:admin permission via Event::TemplatesController and a shared
controller-level guard (EventsController#assert_template_admin),
chosen over an ability-DSL patch since wagons define their own custom
Event ability constraints that wouldn't have picked it up.

Also fixes several related bugs surfaced while building this:
Event#new was appending default question templates on top of a
duplicated source's own questions; Event#duplicate did not reset
template/inherit, so instantiating an event from a template (or
saving one as a template) could silently carry the wrong flag;
list_entries in Event::TemplatesController overrode ListController's
default implementation and silently bypassed Sortable's automatic
sort wrapping, so sorting the templates list did nothing; and the
"Typ" column now sorts by the translated model name instead of the
raw (and for the base Event, nil) type column value.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>