Extract tag bindings starting with ## TAG: tag_name

* Error view: manage missing tags, extra tags, tags enabled with missing strings.
* Global status: add a link in the header to translatestrings.
* Translate strings: display a tag near strings bound to tags.
* lang_update, json, listsitesforlocale: ignore tag_bindings.

On purpose, these are not moved around by lang_update, they live only in the source locale.

It should be safe for [mozilla.org parser](https://github.com/mozilla/bedrock/blob/master/lib/l10n_utils/dotlang.py#L62-L64), not sure about Locamotion.

For testing, you might want to apply [this patch](https://pastebin.mozilla.org/5460056) to the en-GB folder.