Change instance variable to class name for accessing static members

Fix #66
Add `EMPTY_TRASH_DAYS` to system info page

Fix #67
Change the contextual help content for admin screens

Fix #68
Fix JavaScript validation for "Delete by URL" module

Fix #69
Add support for "Delete From Trash" addon

Fix #65
Bumping up version to v5.1 for release
Regenerate pot file for v5.1