This fix is for the experimental user menu. Some `bookmark_reminder` notifications may not be associated with a topic/post (e.g. bookmark reminder for a chat message) in which case the default notification renderer cannot figure out the `href` for those `bookmark_reminder` notifications. This commit teaches the `bookmark_reminder` notification type renderer to fallback to `bookmarkable_url` that's present in the notification data if the default notification renderer doesn't return a `href` for the notification.
Prior to this commit, we had a default Glimmer component that was responsible for handling generic rendering of notifications in the user menu, and many notification types had a custom Glimmer component that inherited from the default component to customize how they were rendered. That implementation was less than ideal because it meant plugins would have to create Glimmer components to customize notification types added by them and that would make the surface area of the API too big.
This commit changes the implementation so there's only one Glimmer component for rendering notifications, and then notification types that need to be customized can create a regular JavaScript class - `renderDirector` in the code - that provides the Glimmer component with the content it should display. We also introduce an API for plugins to register a renderer for a notification type or override an existing one.
Some of the changes are partially extracted from https://github.com/discourse/discourse/pull/17379.
