Add a "prerequisite" tags input (like http://mbenford.github.io/ngTagsInput/) to the item edit screen

On the backlog screen, for items that have prerequisites
1) Add "requirements not met" with a tooltip explaining what needs to be done
2) Fade out the table row so it's easy to see we're not ready

Justification: I spent a lot of time jumping around with items that I couldn't yet do, because I didn't have an MS account or anything.

The feedback of items becoming available will feel like you're progressing faster