Allow attaching a bio (profile text) to a webhook

[hikari-no-yume]

Right now when you click on an “APP” message from a webhook, the profile widget that pops up only shows the webhook display name. It would be very useful to be able to provide a bio text, like for ordinary user profiles:

• This could make webhooks less confusing. The display name already provides some information, but a more detailed description of “what am I looking at” could go in the “bio”.
• For particular applications, it would be especially useful to be able to set the bio on a per-message basis, for providing info like “who specifically triggered this message and when”.