core: frontend: sidebar: allow pinning pages

### Current behaviour

Sidebar order is fixed, which can be messy (especially in pirate mode).

### Expected or desired behaviour

Allowing the user to pin/star/favourite pages to the top of the sidebar would be convenient, especially if it works for Extension service pages[^1].

Pinned pages should be displayed in insertion order, so that a user could conceivably pin every page in the order they want to see them. We could also allow drag re-ordering pinned pages (or just all pages), but that's extra effort that may not be worth it.

[^1]:It may be necessary to support greying out pages that are pinned but not yet loaded / available (both for internal and Extension services, including for if a service changes its name).

We've discussed this feature previously, but we don't seem to have made an issue for it.

### Prerequisites

- [X] I have checked to make sure that a similar request has not already been filed or fixed.