Ask user about deleting uploaded files when removed from message

[alya]

In zulip/zulip#25525, we plan to ask users whether uploaded files should be deleted when removed from a message. When we do this, we should implement a parallel experience on mobile.

References

• Prerequisite issue in this app: compose: Support editing an already-sent message #126
• Server/web issue: Ask user about deleting uploaded files when removed from message zulip#25525

[gnprice]

(This issue was originally filed on zulip/zulip-mobile, which already has the "edit message" feature. This issue is a refinement of "edit message". I've edited it to reflect that in this app we don't yet have that feature (#126).)