Add "Layer Upgrade" UI

[isaisabel]

As a user of ATT&CK, I want to be able to upgrade layers created in older versions of ATT&CK with an intuitive UI.

• This UI should perform the same operations as the update-layers script automatically, and provide notification to the users of the revocations that were followed.
• The UI should notify the user of what objects have changed between the two versions (see also diff_stix.py for other implementations showing changes between STIX bundles).
  • It should highlight changes in two lists, one for objects which have changed which had annotations, and one for objects which changed which did not have annotations. These lists should be shown as adjacent columns or tabs.
  • Changes should be grouped by type:
    • Changes which removed the object (deletion or revocation) and have therefore removed annotations the user had added.
    • Changes to the object content (major and minor changes), which may require updated annotations
    • Object additions, which may require new annotations
    • which objects have been upgraded automatically
  • Possibly allow the user to filter the lists to show only techniques, or all objects (which would be useful if the user was tracking mitigations or APT coverage)

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This UI should take the form of a sidebar adjacent to the Matrix. Clicking on a change or section should select the object/objects within the section where they occur in the matrix, and likewise hovering should highlight objects where they occur in the matrix. Highlights and selections should also work the other direction (where they occur in the matrix they also occur in the sidebar).

For objects which changed that have annotations (in particular cases where the object is no longer in the matrix, e.g revocations and deletions), the previous annotations should be shown on the item so that the user can remap them.

The user should be able to minimize and expand the sidebar if they so choose.