Requests for Comment/Central notice changes

These are a couple of proposals related to central notices, previously from Requests for Comment/Improvements for notification of global discussions where they happened to have failed due to insufficient consensus. Hopefully a more clear consensus can be found in this discussion.

Proposal 1: Central notice duration
Central notices for discussions should last while the discussion is open for people to participate. As in, the central notice would only be removed after the discussion is resolved.

Proposal 2: Make campaign types for central notices
A campaign type can be set for central notice campaigns, allowing users to opt out of specific campaign types in their preferences, specifically in the "Banners" section. Here is a proposal for what campaign types Miraheze should use:
 * Fundraising
 * Surveys
 * Maintenance
 * Requests for Comment
 * Requests for Stewardship
 * Requests for Community Director

To make it clear how to use preferences to opt-out of campaign types, some text instructing people how to do so should be added to central notices.

In technical terms, campaign types are configured with $wgCentralNoticeCampaignTypes in LocalSettings.php.

Include RfGS as a campaign type
Include Requests for Global Sysop in the list of campaign types.