{{t}} helper uses registerBoundHelper

 * use `registerBoundHelper` to register `{{t}}`
 * `{{t}}` no longer supports passing a `tagName`
 * drop support for `I18N_TRANSLATE_HELPER_SPAN` and remove
   corresponding `taglessTranslateHelperSpec.js`
 * tests use `fooBinding="bar"` instead of `foo=bar`, per behavior
   of `registerBoundHelper`
 * mark compatibility with all Ember 1.x
 * remove vestigial `warn`, `isBinding`, `uniqueElementId`,
   and `TranslationView`