Fixed LIB robot user/cert migration.

Added guardian migration for LIB user pipeline perms.

Corrected LIB username.

Fixed translator.py to read JSON for LIB events.

Needed to freeze gracedb for guardian migration.

Changes for new LIB user/pipeline and permissions.

Added migrations for gdb_pe robot cert and 'background' tag. Also added 'background' to the list of blessed tags in the settings module.