Previously, there was a 50% opacity loading overlay with a spinner,
which was awkwardly replaced with the empty editor overlay when it
loaded. Then when article data loaded, it was again suddenly placed
inside the overlay.

Now, upon clicking one of the edit buttons:
* Immediately:
  * Loading overlay appears with 50% opacity
  * Loading bar appears. It animates until the editor is loaded.
* After 100ms delay:
  * Page scrolls to align the clicked section (or the top of the
    article) with the top of the viewport (400ms)
  * Section edit links and collapse/expand buttons fade out (400ms)
* When the editor code is loaded:
  * Toolbar slides into view (250ms)
* When the editor code and article data is loaded, and the scroll
  animation finishes:
  * Loading bar disappears
  * Editor replaces the loading overlay

Technical notes:
* When the editor code is loaded, loading overlay is replaced in-place
  by the editor overlay, which uses 50% opacity until the article data
  is loaded. This forces two awkward things:
  * Loading bar has to be placed in a separate overlay on top of both
  * We have to handle two cases when loading is cancelled by the user
* The scroll animation is partially implemented using CSS transforms,
  to allow smooth animation using CSS transitions.
* No animations are done when switching from wikitext to visual editor.

Bug: T210630
Change-Id: I6dc56495cb1b24ce0097d78f5e4ee0df3a3c1ee4