This moves the "This site was just updated" modal asking the user if they want to refresh into a subtle prompt that slides down from the header.
Also in this PR I've added a helper to publish message bus messages in JS tests. So instead of this:
```javascript
// Mimic a messagebus message
MessageBus.callbacks
.filterBy("channel", "/global/asset-version")
.map((c) => c.func("somenewversion"));
```
We can have:
```javascript
publishToMessageBus("/global/asset-version", "somenewversion");
```
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