Completely translate the app/views/projects/_issuable_by_email.html.haml partial

Currently only generated email from the email link is translated, the modal itself is not.

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The following discussion from gitlab-ce!21247 should be addressed:

• @godfat started a discussion: (+4 comments)

  I think we need to split this. We cannot translate issuable_type when it could be either an issue or a merge request. Does this work..?

  _("Enter the %{#{name}} title") % { name.to_sym => name }

  I guess not :o Then we could only repeat it:

  issuable_type == 'issue' ?
    _("Enter the %{issue} title") % { issue: name } :
    _("Enter the %{merge_request} title") % { merge_request: name }

  Or do we need to pass the value anyway?

  issuable_type == 'issue' ?
    _("Enter the issue title") :
    _("Enter the merge request title")