Roll results change as new emotes are added

[noodlebox]

When new emotes are added, previously rolled emotes should not change. Emote data now includes a creation date, making it possible to build a list of the emotes that existed when an older message was originally sent.

Messages before the emote epoch (2017-02-08 or some arbitrary date between that and release) should use all emotes created on or before that date.

Since fixing this will likely change many older roll results anyway, it may finally be worth including this old fix for some ugly roll math: 3448e80.