Load data via STIX 2.1 Collection Index

[clemiller]

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

[sonarcloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
2.8% Duplication on New Code

See analysis details on SonarCloud

[erinehall]

LGTM!