- Moved aboutaddons.html at the root of the about:addons page
- renamed aboutaddons.js `initialize` global function to `initializeView` (and renamed `show` and `hide` global functions accordingly)
- Simplify initial load logic a bit more:
  - remove gCategories
  - moved responsability of the CategoriesBox initialization to the `initializationView` function
  - replaced initialization logic based on `gPendingInitializations` and `notifyInitialized` with
    a `promiseInitialized` global resolved when `initializeView` method has been called and its return value resolved
- Fix test helpers and test failures
  - InlineOptionsBrowser: take into account that browser-custom-element does opt-in the delayedConnectedCallback behavior
    and browser.loadURI may throw if called while the document is still loading.

Differential Revision: https://phabricator.services.mozilla.com/D96472