Expand problematic PFP on-join screening

Brief:

Expand entrant screening of profile pictures to include third-party "default" avatar collections and other categories that may correlate with members being nuisances or otherwise unwanted.

Assets

Pending implementation of #451 for DB-based asset storage, these would be SHA-256 hashes added to bot/constants.py. We do not have to wait for #451's completion before working this task.

1. "Default" avatars that are third-party variants of the stock Discord defaults such as from this collection and across the net (Google Images and Site searches, manual collection from public servers, etc.)
2. Meme/troll images found to be used on public servers.

Implementation Detail

Requirements:

• At least implement the first collection to supplement the current avatar scanning policy.

Nice to haves:

• The latter collection would require separating the "pfp" policy into a further subset, as I think it's utility may be debatable given the demographic of the guild that cables is employed on.