* FEATURE: Content custom summarization strategies.
This PR establishes a pattern for plugins to register alternative ways of summarizing content by extending a class that defines an interface.
Core controls which strategy we'll use and who has access to it through the `summarization_strategy` and `custom_summarization_allowed_groups`. It also defines the UI for summarizing topics.
Other plugins can access this summarization mechanism and implement their features, removing cross-plugin customizations, as it currently happens between chat and the discourse-ai plugin.
* Group membership validation and rate limiting
* Work with objects instead of classes
* Port summarization feature from discourse-ai to chat
* Rename available summaries to 'Top Replies' and 'Summary'
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