compose: Implement disabled appearance for edit-message "Save" button

[chrisbobbe]

When we first implement the edit-message compose box (#126), we plan to skip a feature that the regular compose box has, because we don't have a Figma design for it.

The regular compose box gives the "Send" button a faded appearance when there are validation errors. Compare:

Validation error (content too long)	No validation error

We should do the same with the "Save" button in the edit-message compose box:

See ZulipWebUiKitButton in the code, and the Figma frames linked from there, which don't include a design for a disabled state.

Tapping the "Save" button still gives a dialog explaining the validation error(s), just like the regular compose box's "Send" button, which mitigates the bug a lot.