Add dynamic variable substitution to conference descriptions

[sikesbc]

What this PR does:

• Adds dynamic variable substitution (e.g. {conference}, {conference_start_date}, {venue}) to conference description and registered attendees message markdown fields. Reuses the existing email template variable system so organizers can embed conference data that auto-populates at render time.

Include screenshots, videos, etc.

• N/A: backend feature, admin form now shows available variables below description fields.

Who authored this PR?
@sikesbc

How should this PR be tested?

• Edit a conference description in the admin panel and add placeholders like {conference} or {conference_start_date}.
• View the conference splash page and confirm the variables are replaced with actual conference data.
• New specs in spec/services/email_template_parser_spec.rb and spec/helpers/format_helper_spec.rb cover the parser with nil user and the new markdown_with_variables helper.

Are there any complications to deploying this?

• No migrations or data changes. The EmailTemplateParser now accepts an optional user parameter (defaults to nil), which is backwards compatible with all existing callers.