Some available languages lack translations for notification mails. Now it
tests for two languages which is certain to have required translations:
German and English.
German is the language the current project owner, Eugen Rochko speaks, and
providing English translations for new messages is de facto mandatory.
* Dont use raise_error by itself (avoids warning)
* Add coverage for AccountFilter
* Improve coverage and refactor for Subscription#lease_seconds
* Improve coverage and refactor for NotificationMailer
* Simplify assignment of min/max threshold on subscription
Akihiko Odaki
Matt Jankowski
Jessica Stokes
Eugen Rochko