Make additional pages translatable

With this, I think the quest to make this tool translatable is
functionally complete; there are only a few hard-coded bits of English
left that I don’t want to make translatable yet.

The biggest chunk left is in oauth-callback-without-request-token.html,
which I’m leaving alone for several reasons:

• The markup is a bit complicated, depending in part on whether you’re logged in or not.
• This page is shared across several tools (and included in cookiecutter-toolforge); arguably any translations for it should be shared as well, and I’m not yet sure how to do that.
• The page should virtually never be shown to normal users; it’ll mainly be seen by tool developers (i.e., mainly me), where translation is less of a priority.

Maybe I’ll make this page translatable eventually, but not soon, I
think.

Also sort the _allowed_html_elements, I don’t think the previous
(implied) order by whenever the elements were added made much sense.

Bug: T363626