Make blacklist and irredeemables optional

Currently, the blacklist and muted accounts in hivemind are hardcoded:

BLACKLIST_API_URL=https://blacklist.usehive.com

MUTED_ACCOUNTS_URL=https://raw.githubusercontent.com/hivevectordefense/irredeemables/master/full.txt

Although highly recommended to use those lists, there should be an option to disable using them. For example if https://blacklist.usehive.com goes down. We had such an incident when migrating https://blacklist.usesteem.com, where hivemind froze because the site was unreachable.

Suggestion: Passing an empty string to BLACKLIST_API_URL and MUTED_ACCOUNTS_URL would override the hardcoded defaults and disable them, while making sure hivemind skips those lists when querying the database.