Define edit permissions for closed topics and forums

[johnjamesjacoby]

bbPress currently treats closing a topic or forum primarily as preventing new content, while users who satisfy the normal edit capabilities may continue editing existing topics and replies.

The intended contract should be made explicit and enforced consistently. In particular:

• Should closing a topic prevent edits to its existing topic and replies, or only prevent new replies?
• Should closing a forum prevent edits to its existing descendant topics and replies, or only prevent new content?
• Which roles and capabilities may override either restriction, including Keymasters, Moderators, and per-forum moderators?
• Should the same rules apply through theme compatibility forms, wp-admin, REST, XML-RPC, and custom integrations?

Any change needs to preserve established extension points where practical and include capability tests for authors, Participants, Moderators, per-forum moderators, and Keymasters. The resulting behavior should also be documented so closing content has a predictable meaning for site owners and integrations.