Allow ATT&CK Navigator links to be configurable

[isaisabel]

As a user of the ATT&CK Website, I want to be able to set links to the ATT&CK Navigator (e.g the buttons on Matrix pages, the technique usage links on group and software pages) to point to a local instance. Currently they will always lead to the official MITRE hosted instance.

This will allow users with custom STIX to host a Navigator instance with the custom STIX alongside an instance of this website with the custom STIX.

We will need to provide config options for both the enterprise and mobile instances.