Turn off gather-from-revisions by default

[andrewhead]

Is your feature request related to a problem? Please describe.
If we let people explore the versions of slices that produced results by default, the tool may be unusable for long sessions. This is because outputs can take up a lot of memory and, when the notebook is saved, storage. To enable people to explore versions of slices with their output, we need to keep every versions of every output produced.

Describe the solution you'd like
In the settings menu for the plugin, include a configuration option for turning on the "Gather from Revisions" action. Have a note next to the option that tells the user that if the feature is enabled, all outputs ever created will be saved to a notebook, which has the potential for increasing the size of the notebook many times. Add documentation for how to do this to the README.

Describe alternatives you've considered
Leave Gather to Revisions in by default. That said, our usability study suggested the tool is most useful for slicing and gathering code that was executed out of order, not gathering from revisions. I also feel that this feature, while exciting, hasn't yet reached the form where it will be obviously useful and easy-to-use in common cases where people want to use it. So for this tool to be useful, perhaps the feature of gathering from revisions should be opt-in only.

Additional context
N/A