Translations: Switch to Transifex format (3.1)

Same as 3.0 PR with https://github.com/silverstripe/silverstripe-cms/pull/820, same master and translation files, but different templates.

- Based on new (last) translation download from getlocalization.com
- Removed untranslated strings. Getlocalization started including those at some point
which is highly annoying, unnecessary and breaks the new transfix system,
since it'll mark all of the english strings as actual translations
- Avoid dots in entities. It confuses the Transifex YML parser
- Removed some locales unknown to Transifex which didn't have any translations anyway
- Removed "lolcat" locale, uses custom notation (en@lolcal)
  which SilverStripe's i18n system can't handle
  (needs mapping from SS naming to Zend naming)
- Renamed "Te Reo/Maori" locale from "mi_NZ" to "mi" (Transifex/CLDR notation)
- Namespaced all entities used in templates (deprecated usage)
- Converted dots to underscores where template filenames are used for namespaces,
since Transifex YML parsing handles them as separate YML keys otherwise
- Removed whitespace in entity names, SilverStripe i18n can't handle it
- Only allow selection of locales registered through i18n::$all_locales to avoid
  issues with unknown locales in Zend's CLDR database