Enable source mode in RTC through a configuration flag

[scofalik]

📝 Provide a description of the improvement

Currently, source editing plugin does not work with real-time collaboration. They cannot be put together in the online builder. If an integrator adds both plugins to the editor, when source editing is used, it will display an error.

It's done like this for a reason. When the source editing is on, the source (HTML) is not updated as changes happen in the document during real-time editing. After submitting the edited source, this will lead to overwriting all changes that were done in the meantime. This is a non-obvious interaction (limitation), and it may surprise a user and lead to data loss. That is why we decided to prevent it.

However, there are reasonable cases where it makes sense to allow the user to edit the source. If only one user has joined the document, there's no risk. If the users are "power users" that know about the limitation, then they can use the source mode only when they will feel it is safe. And if someone uses sorce mode, they often are a "power user".

This is why we decided to allow for using source mode together with RTC.

The first step for this will be to add a configuration flag that will need to be explicitly turned on to allow using these features together. Otherwise, the editor will throw an error and the link will lead to a description explaining what is going on. After switching the flag, it will be integrator who takes responsibility for possible data loss.

In another ticket, we will introduce user-facing solution for the problem, that will inform the users directly that possible data overwrite is going to happen.