mw.ViewPageTarget: Make 'review' step optional in save flow

Summary:

Instead of having a button "Review and save" that opens with a
diff and leads the user to the "Report a bug" and "Save page"
slides respectively, make it more like the default EditPage.

There is now a "Save page" button that opens with the save
form with a button to "Review changes" (diff) or "Save page".

The "Report a bug" slide has been unlinked from the UI and is
no longer accessible for now.

As a result of the UI no longer requesting a diff upfront this
also means we will no longer detect "nochanges" event (when it
turns out the submittted revision matches the latest version).

This is unfortunate as it was a nice feature to detect it
before the user spends time writing the edit summary) but it
is the same as how the default EditPage works.

Changes:

Improved interface messages.

Adapted "nochanges" caption to the new context (it is no
longer shown when clicking "Save page", it is now shown as a
result of clicking "Show changes").

Now that the "save" slide is accessible from multiple paths
it is needed to keep track of slide changes in a history
array. Previously the slide tree was 1 level deep with
everything descending from "review". Now it starts at "save"
and can go in multiple directions including a loop from
save>review>save. We also need to toggle the "Prev" button
based on history instead of based on whether or not we are
on the "first" slide.

Hid the "saveDialogReviewWrongButton" from the review slide.
We're approaching wider launches and this will not scale to
a wider audience.

Bug: 49258
Change-Id: I90de95f6337eeddd794b75d56543d8d152421a6f
Merge "mw.ViewPageTarget: Make 'review' step optional in save flow"