[ui/ux] design add-on management ui/ux

Simply what it says on the package:
review the UI/UX implications the new add-on model will have and prepare the designs.

Some immediate thoughts:

• user will select add-ons from some self service page (at least for standard offerings) - the most obvious thing
• as the acceptance criteria for the epic state - just subscribing to add-ons is good enough for i1; we can handle deletion manually for now
• user will also have to know what subscriptions they're on currently
• user must clearly know which features are available; assume we'll handle 'allowed' addons via RBAC headers as usual (to be designed), figure out how to signal feature availability at least for the mender shell.

You'll figure it out best, just throwing it out there.