V8 implementation of Psych-DS and metadata module package

[vzhang03]

This is an update to the previous Psych-DS PR to work for v8 rather than implementing for v7. The main feature is the metadata module which is implemented as a package allowing for metadata to be generated separately from an experiment.

This module depends on the packages on unpkg to be updated with the new metadata format in this related PR as well as the writing of the versions in the same PR. While the fetching has not been tested completely due to the dependence on the packages being published to unpkg, the backend server simulates some of this behavior. Still needs to implement more detailed error checking.

Functionality included within the metadata module:

• Can initialize metadata module with all the default variables.
• Can pass into a JsPsych-generated data file to have the module scrape descriptions and generate "default metadata" with all plugin information.
• Can pass in a dictionary to customize the metadata according to researchers.
• Can save metadata as a json file.

Misc features/comments:

• Included the backend server for testing and replication, but can be deleted for the final version.
• Due to how the variable mapping is written should document how to work with the nested mappings ("descriptions" within variables), but could advise user to edit manually.