- Reduce time-to-digest from 20 to 7 days
- Fetch mentions starting from +1 day since last login
- Fix case when last login is more recent than last e-mail
- Do not render all mentions, only 40, but show number in subject
- Do not send digest to moved accounts
- Do send digest to silenced accounts

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.

* Localize date in digest

* Cover NotificationMailer more

* 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

A couple of places were using "reblog" rather than "boost" - this updates them to match the web UI