[Discord] "Preemptive" rate limit check

[torlenor]

The Discord bot shall look at the X-RateLimit Header entries to prevent hitting the rate limit in the first place. The current handler only looks at the Headers when we are already rate limited and prevents sending events then.

Example of the header entries:

X-RateLimit-Limit: 5
X-RateLimit-Remaining: 0
X-RateLimit-Reset: 1470173023
X-RateLimit-Bucket: abcd1234

For further details see https://discordapp.com/developers/docs/topics/rate-limits#rate-limits