Allow the site to be generated using data from a TAXII server

[isaisabel]

As a user of the ATT&CK website, I want to ensure that my local copy of the data is up-to-date. By connecting to the ATT&CK TAXII server, I can make sure every build uses the latest data.

Allow the user to build using data from the TAXII server instead of local files in data/stix. The TAXII server configured (default should be the ATT&CK TAXII server) should also be used for documentation generated for #137. The configuration for this can be formatted like it is on the ATT&CK Navigator.

[jondricek]

Closing due to inactivity - may re-open if further interest in this topic is brought up in the future