This modernizes an old part of the build system to not require
build-time localization at all. That's generally preferable.
The most significant changes to the in-product functionality is to
make import localize HTML so that we can use Fluent's `data-l10n-id`.
The locale used is the user's current locale. This is different than
the existing approach, which always uses the build-time (repack)
locale. I believe this is a strictly superior user experience and it
may lead to future improvements where-in the default bookmarks become
truly dynamic and vary with the user's chosen locale rather than being
point-in-time decisions.
I tried to restrict these changes to only applen when we import the
default bookmarks, but I think the various layers of flags no longer
achieve this restriction in practice and the formatting and
localization will apply to all imported `bookmarks.html` files. Since
we don't anticipate (nor ourselves write) these new things in
(respectively, to) `bookmarks.html`, and the file is already
user-controlled, I don't think this exposes any meaningful change in
functionality (or in security surface).
Some notes:
1) There's no migration of `.inc` -> `.ftl` because this is the lone
`.inc` file.
2) I elected to prefix all strings with `default-bookmarks-`, since
the existing names were very short and likely to collide (now or in
the future).
3) I elected to change the HTML file name for easier searching.
4) Since the `default-bookmarks.html` file is product-specific and the
existing tests are in `toolkit/`, I elected to not test the file
directly in automation.
5) We removed the explicit locale (or equivalent `%LOCALE%`) since
Mozilla properties will redirect to the appropriate language
automatically.
Differential Revision: https://phabricator.services.mozilla.com/D135816
381dd7c640