Support deleting a message completely

[chrisbobbe]

We should add an option to the message action sheet to delete the message completely, with the label "Delete message". This feature: https://zulip.com/help/delete-a-message#delete-a-message-completely

(Editing your message to clear its content is already supported: if you're still within the edit-message time limit, open the action sheet, tap "Edit message", clear the content, tap Save.)

API endpoint to use: https://zulip.com/api/delete-message

We should check the appropriate permissions/time limit to decide whether to show the button.

[chrisbobbe]

This came up in beta feedback today: #mobile > Flutter Android 0.0.30 feedback @ 💬

[timabbott]

This was requested here: zulip/zulip-mobile#5926.

I expect that this is likely to be fairly easy once we support checking group-based permissions -- in theory it should be just a single button that does an API call, maybe with a confirmation dialog.

[chrisbobbe]

Checking group-based permissions is

• Implement group-based permission checks #814

That'll be some work; it may be wise to ship a "Delete message" button without that check, initially. (You'd just get an error from the server if you don't have permission.)

[emilysutherlin]

This came up today from a Zulip Cloud Standard user, noted as "a minor inconvenience but nice to have".

[alya]

Requested in #mobile > share to zulip @ 💬.