Add ephemeral option to /wiki

[Eisenwave]

Sometimes, a user wants to post a wiki article just to look up information, not to show it to others. For example, a user may want to view the compiler support article just so they can look up information themselves or to grab an individual link and share it with others.

It is possible to do this by going to #bot-spam and posting the embed there, but this is not great UX because it requires a channel switch. It may be more convenient to add an optional ephemeral argument to /wiki to toggle this.