Update the channel publishing logic to allow the staging tree to be used as the base for publishing

[rtibbles]

• Update the channel publishing logic to optionally allow the staging tree to be used as the base
• Ensure that the default behaviour is still the main tree
• When doing this, do not increment the channel version
• Update the channel publishing logic to create a database called <channel_id>-next.sqlite3 when the staging tree is used for publishing
• Write tests to cover these scenarios

[taoerman]

Hi @rtibbles! I am Eddie Liu. Can I work on this task?

[rtibbles]

Assigning as part of prep for associated GSoC project.

[rtibbles]

Fixed in #4980