Allow publishing of a staged channel as `next`

[rtibbles]

Desired behavior

When a channel is staged for deployment by the ricecooker library, the only way to test the channel in a veridical environment is to publish the channel and import it to Kolibri. This involves deploying the channel and then publishing, which somewhat defeats the point of having a staged channel for quality assurance purposes. This is particularly problematic if the channel is already being widely used, and a potentially flawed update might be published.

While there are existing workarounds for this, such as staging the channel to an alternative channel, this adds additional burden to the content integration and quality assurance processes.

Instead, it would be desirable for a staged channel to be 'published' as the next version of the channel.

Requirements

• [x] Prerequisite task: https://github.com/learningequality/studio/issues/4470
• [x] https://github.com/learningequality/studio/issues/4977
• [x] Create a new change type for creating a 'next' publish
• [ ] Add a button on the staging tree frontend view that reads "Publish draft" - it can be in a similar location to the usual publish button - which will initiate the task using the same change event mechanism that publishing and deployment already use.
• [ ] Track the task progress in a parallel way to the publishing task.

Value add

Allow easier testing of staged channels

Possible tradeoffs

This adds a small amount of additional complexity, and will also require some tweaks to Kolibri to make this completely usable.