Some of the changes in this PR are extracted from https://github.com/discourse/discourse/pull/17379.
Similar to the bookmarks tab in the new user menu, the messages tab also displays a mix of notifications and messages. When there are unread message notifications, the tab displays all of these notifications at the top and fills the remaining space in the menu with a list of the user's messages. The bubble/badge count on the messages tab indicates how many unread message notifications there are.
Some of the changes in this commit are extracted from https://github.com/discourse/discourse/pull/17379.
The bookmarks tab in the new user menu is different from the other tabs in that it can display a mixture of notifications and bookmarks. When there are unread bookmark reminder notifications, the tab displays all of these notifications at the top and fills the remaining space in the menu with the rest of the bookmarks. The bubble/badge count on the bookmarks tab indicates how many unread bookmark reminder notifications there are.
On the technical aspect, since this commit introduces a new `bookmark-item` component, we've done some refactoring so that all 3 "item" components (`notification-item`, `reviewable-item` and the new `bookmark-item`) inherit from a base component and get identical HTML structure so they all look consistent.
Internal tickets: t70584 and t65045.
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.
