[FEATURE] Send ephemeral "do you want this to be your new fetch image" in showcase

[elkowar]

Checklist before submitting an issue

• I have searched through the existing closed and open issues and made sure that this is not a duplicate

Is your feature request related to a problem? Please describe.

Some users may be surprised by showcase posts overwriting their fetch image

Proposed feature

It would be neat if robbb would send an ephemeral response to showcase posts (if that's possible? lol) that asks if the user wants it to be their new fetch image.

Additional context

Probably not possible right now, as discord doesn't allow ephemeral responses to normal messages -- only application interactions