I18n health warnings (#1949)

* Rename admin.domain_block to admin.domain_blocks in prep for i18n improvement

* Use implicit controller/action path for i18n in admin/domain_blocks

* Add DomainBlock#accounts has_many

* Avoid i18n health warning for `en` locale by using symbol scope with :count

* Remove unused i18n key: plaintext_secret_html

* Remove unused i18n key two_factor_auth.warning

* Remove final will_paginate i18n keys

* Remove unused key two_factor_auth.recovery_codes

* Remove unused key: admin.reports.comment.none

* Remove unused reports. i18n namespace (moved to admin.reports)

* Ignore keys from locales which override activemodel and activerecord errors

* Revert "Remove unused key: admin.reports.comment.none"

This reverts commit 350ef2685fadc069e619bb6d1066190de195d942.

* Update i18n key reference to match moved location

* Add missing `en` keys to i18n

* Tell i18n-tasks to ignore missing attributes that dont need overwriting

* Add i18n-tasks unused to travis