Reset $extensionKey when loading each file. This puts the onus on developers
to set the extensionKey once in each file using {assign} or {tsScope}, but
it prevents the extension key from leaking unintentionally to other files,
and it allows developers to handle weird situations explicitly.

(Example weird situation -- using one extension to override another
extension's template; or having multiple extensions share a common
translation key.)

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* CRM-13580: Set translation domain implicitly in extensions
  http://issues.civicrm.org/jira/browse/CRM-13580