Localization in progress improvements

What this PR does:

* Adds new maintenance page under /locale/docs/localization-tag/inprogress (only makes sense for non en-US). And /locale/docs/localization-tag/inprogress/ is the only l10n tag present in Kuma right now.
* Exposes "localization_tags" to the Kuma JSON API.
* Fixes bug 996499: The initial value of the "Localization in progress" checkbox in the RevisionForm is checked by default, if you are on the $translate route (= you create a new translation, you don't edit one).
* Adds a banner "This article is in progress of being translated." to the top of the document if it is the case.

Note that I know that the listing methods are not ideal as they are, I filed https://github.com/mozilla/kuma/issues/2190 some time ago.