Crawler doesn't crawl unless Read Message History perms are explicitly granted

[robstolarz]

Repro: point statbot at a guild containing channels that don't have Read Message History explicitly granted to a bot user. The crawler will ignore those channels.

Consider https://github.com/strinking/statbot/blob/81137108fa9bb4011936c79f3ee5288967804505/statbot/crawler.py#L159

Replacing this conditional with True will cause the crawler to start indexing those channels again (although it will then fail on channels it can't read).