Load data via STIX 2.1 Collection Index

Summary of changes:

Updated Navigator to load data via the STIX 2.1 Collection Index
- The URL to the collection index is specified by the collection_index_url property in config.json
- Removed the hardcoded list of ATT&CK versions from the versions property in config.json
- Continues support for the versions property for loading custom versions of the dataset, integration with the ATT&CK Workbench, loading content from a TAXII server, and for offline hosting. This property is now disabled by default and provides an example setup for a custom dataset.
Added support for loading content from a TAXII 2.1 server. Support for TAXII 2.0 is continued.
Improved error handling when there is an issue loading the configuration file. Navigator will now show an alert to inform the user of errors, rather than failing silently.
Updated the README and the CHANGELOG

Resolves #607, #277, and #398

Supersedes #586

mitre-attack / attack-navigator