Translations: Switch to Transifex format (3.1)

See https://github.com/silverstripe/silverstripe-framework/pull/2291 for 3.0 PR. 
Same master and translation files, but different templates, and slightly different implementation for the `i18n::get_esisting_translations()`.

- 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
- Renamed "lolcat" locale from "lc_xx" to "en@lolcat" (Transifex/CLDR notation)
- 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