Fix reports failing on big messages

[hyperneutrino]

Reports currently fail if the message is too large (reported by a member). This surfaced due to an image with 6 images in the memes channel. We should find a better implementation in a future rewrite but a naive truncation is better than nothing for now.

[MysticMia]

it is technically possible to add multiple fields each limited to 1024 chars
but it probably doesn't matter given that it's for a reported message (and the message link will be provided anyway)
plus you'd run into issues with nitro users sending 4000 chars; with embeds having only up to 6000 chars max.