Refactor domain dropdown from sidenav into tree-items

[isaisabel]

As a user, I want it to be easy to navigate around the ATT&CK website including across domains.

Objects with domains (e.g techniques, tactics and mitigations) have a domain selection dropdown button above the sidenav. We should refactor this to be part of the sidenav itself, thereby allowing users to access the object domain lists of other domains.

For example, on the mitigations page, we would have:

Mitigations
Enterprise
    - Account Use Policies
    - Active Directory Configuration
    - ...
Mobile
    - Application Developer Guidance
    - Application Vetting
    - ...

The domain-section headers can serve as links to the domain overview pages for the object type. This new design will be more consistent with the matrices page sidenav.