Add subtechniques to Navigation and Technique lists

[isaisabel]

As a user of the ATT&CK website, I want to be able to see subtechniques in the navigation elements.

Add subtechniques to the techniques sidenavs under their parent technique, and in the master techniques list nested under their parent technique.

Technique List Mockups

Note: using fake data for names, IDs and descriptions.

Option 1 (full rows)

1.1

1.2

Option 2 (indented rows)

2.1

2.2

Option 3 (nested rows)

3.1

3.2

[isaisabel]

Closing because it has been implemented on the subtechniques branch.