Layer Upgrade UI

[clemiller]

Implements the Layer Upgrade workflow in the sidebar to upgrade layers created in older versions of ATT&CK.

• Displays technique changes that occurred between the two versions
• Allows users to map annotations from the previous version to the current version
• Techniques with tactics that do not map one-to-one between versions have a drag & drop interface
• Each step can be filtered to show only techniques which have annotations
• Provides links to attack.mitre.org for both versions of each technique

Resolves #181

[isaisabel]

This is a fantastic improvement and I really only have minor comments at this point. There is really only one behavioral change, the rest is just documentation improvements.

[isaisabel]

The changes all look good here, I have no other comments on the layer upgrade UI.

I have discovered one comparability issue with the current development branch: the "search & multiselect merge notification" added in #336 could appear alongside the layer upgrade sidebar if the first thing a user does when they open the v4.4 Navigator is upgrade a layer.

The merge notification popover should not be shown if the user is upgrading the layer. Adding an extra check to this statement to determine if the layer upgrade dialog is present should produce the correct behavior: https://github.com/mitre-attack/attack-navigator/blob/develop/nav-app/src/app/search-and-multiselect/search-popover-notification/search-popover-notification.component.ts#L79

With that compatibility issue resolved this PR is suitable to merge.