message_edit: Use `disabled` instead of `readonly` to disable controls.
message_edit: Use `disabled` instead of `readonly` to disable controls.

Currently when a user does not have the permission to edit the topic/content
of a message, the edit UI/view source UI correctly shows a greyed
out topic/message-content input field, however these fields incorrectly have a
click behavior, so to fix this we now would want to use `disabled` prop instead
of `readonly` attribute as `readonly` controls can still function and are still
focusable whereas disabled controls can not receive focus and are unclickable.

Fixes #22565.

zulip/zulip,rht/zulip,zulip/zulip,zulip/zulip,rht/zulip,andersk/zulip,rht/zulip,zulip/zulip,rht/zulip,andersk/zulip,rht/zulip,andersk/zulip,andersk/zulip,rht/zulip,rht/zulip,andersk/zulip,andersk/zulip,andersk/zulip,zulip/zulip,zulip/zulip,zulip/zulip