Add an optional ILocalizer parameter to the Codex constructor that is
actually optional (doesn't throw a deprecation warning if omitted). This
parameter is ignored, it's just there to ease the migration to 0.8.0.

Make the $attributes parameter to the HtmlSnippet constructor optional.
It's going to be removed in 0.8.0.

Bug: T434742
Change-Id: I30f79ac2465fed955cb62fb50640b48219111a0f