Use "language_code" instead of "code" for language data.

This matches the responses we get back from the API.
SubtitleList cleanup

Most of this is in preparation of supporting editing a new language
with no existing data.

Don't try to load the initial language data if versionNumber = "".

Don't store versionNumber in SubtitleListController.  We don't use it,
and we won't have a version number if we're editing a new language.

Removed weird timeout code.  We don't need it if we call
SubtitleListFinder.register() early on in the subtitleList directive.

Handle no reference languages a little better.

Don't store version in the LanguageSelectionController.  We don't
actually use it, and we won't have it if there isn't a reference
language to use.
Fixes to make things work if we don't have an initial version

Initialize DFXP to empty content if there isn't an initial verison.

Fixed exception in indexOfFirstSubtitleAfter() if syncedCount == 0.

Fix for updateSyncHelpers when we haven't got the "will-sync-changed"
signal yet.