Discord's Slack-compatible WebHook endpoint has a limitation that Slack

does not. If a field value is over 1024 characters, Discord will refuse

to post the message, responding with a very unhelpful error payload.
Through some trial and error, it appears that they limit the length of

any field value to 1024 characters.
BaseChatIntegration will now pass any field value text through

format_field_text(), which is normally a no-op. In the case of

Discord, the text is truncated and ellipsized if over 1024 bytes.
This should fix posting updates with long descriptions.

Unit tests passed.
Ran some simulations with known failing review requests, and

successfully posted them to Discord.