Add filter for Groups for specific domains

[jondricek]

Right now if you go to the Groups page on the website (https://attack.mitre.org/groups/) you see the whole list of Groups that is tracked by ATT&CK. It would be nice to be able to filter the list by domain such as Enterprise/Mobile/ICS. There are some additional hurdles not only with the technical aspect here but what the right approach as to how we want to display the information. Mostly opening this issue as a starting point, not throwing in all the details here yet.