Feature - conference duplication

[zhouyijun111]

What this PR does:

This pull request implements the Conference Duplication feature (Iteration 2): a Duplicate button on the conference dashboard that takes the user to the “new conference” form pre-filled from the selected conference, with title, short title, and dates left for manual input. On create, it copies registration settings, email settings, event types, tracks, rooms, tickets, venue, sponsors, and custom CSS from the source conference, and excludes user-submitted events and attendee data.

Files changed and purpose

File	Purpose
app/views/admin/conferences/show.html.haml	Add a Duplicate button next to the dashboard title; links to the new conference form with duplicate_from set to the current conference.
app/controllers/admin/conferences_controller.rb	new: When duplicate_from is present, build @conference from the source (description, timezone, limits, etc.) and set @duplicate_from_source. create: When duplicating, merge source attributes (description, custom_css, etc.) before save, then call @conference.copy_associations_from(source) after save to copy related data.
app/views/admin/conferences/new.html.haml	When duplicating, show an alert and a hidden field duplicate_from so the form posts the source conference identifier back to create.
app/models/conference.rb	Add public method copy_associations_from(source) to copy registration_period, email_settings, venue+rooms, tickets, event_types, tracks, difficulty_levels, sponsorship_levels, and sponsors; no copying of events or registrations/ticket_purchases.

Include screenshots, videos, etc.