Add Conference Dynamic Variable Substitution in Markdown

[zhouyijun111]

Summary

This PR adds conference-scoped markdown template variables to splash-page content and improves discoverability of available variables in admin.

Implemented

• Added ConferenceMarkdownTemplate service to interpolate {token} placeholders for conference-facing markdown.
• Reused existing email-template substitution style and added conference aliases:
  • {conference_name} -> {conference}
  • {short_name} -> {conference_short}
  • {cfp_deadline} -> {cfp_end_date}
• Added fallback replacement for known keys so optional empty values do not leave raw {...} tokens in rendered output.
• Added conference_template_markdown helper and integrated it into app helpers.
• Updated splash rendering to apply interpolation before markdown for:
  • conference description
  • registered attendees message
• Updated admin conference form variable-help UI to email-style clickable blue links (Show available template variables).
• Fixed missing-splashpage redirect flow in ConferencesController#show to avoid nil splashpage rendering path.
• Added regression/service specs for interpolation behavior and redirect behavior.

Changed Files

• app/services/conference_markdown_template.rb
• app/helpers/conference_template_helper.rb
• app/helpers/application_helper.rb
• app/views/conferences/_about_and_happening_now.haml
• app/views/admin/conferences/_form_fields.html.haml
• app/controllers/conferences_controller.rb
• app/views/conferences/_header.haml
• spec/services/conference_markdown_template_spec.rb
• spec/controllers/conferences_controller_spec.rb

Test Plan

• bundle exec rspec spec/services/conference_markdown_template_spec.rb
• bundle exec rspec spec/controllers/conferences_controller_spec.rb:30
• Manual check: placeholders in conference description render correctly on /conferences/:short_title
• Manual check: optional missing values do not leave raw tokens
• Manual check: admin variable-help links are visible and toggle correctly

Screenshot

Example: Description + Corresponding Splash-Page