i18n: Allow multiple translations of "Open"

The software defines at least three contexts for the phrase "Open"

  * The name of a ticket queue (Open tickets)
  * A status of a ticket (Open / Closed)
  * An action to be performed (Open a ticket)

References:
(Crowdin login required)
http://i18n.osticket.com/translate/osticket-official/148/enus-de#2915
i18n: Allow pluralization of related ticket links

The terms `%d open|closed tickets` need to be made plural so that the terms
`open` and `closed` can be properly translated.

References:
(Crowdin login required)
http://i18n.osticket.com/translate/osticket-official/148/enus-de#3735