Avoid sending no-op message edits to server

[neiljp]

Currently there are cases when editing a message, where we send details to the server of a message edit, even if the topic or content have not changed. These 'no op' edits are not currently identified by the server and can lead to unexpected no-change entries in message history.

After zulip/zulip#16317 and zulip/zulip#17193 these should not be a problem and filtered from the server side, but this will remain an issue for running against older servers until the release with these PRs is our minimum supported version.

Even when those server fixes are in place, the checks added as part of resolving this issue will contribute to allowing

• minor network optimizations
• potentially hooking UI alerts regarding trying to send an edit with no changes

[zulipbot]

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