REVIEW ONLY: Install update workflow

This is review only for now. This is code complete and the unit tests pass for prompting the user as needed regarding updates and installing the updates. However,

1. it may not yet handle error cases when installing properly (error cases should actually be relatively rare, but they should at least display properly)
2. it has not been human-tested. *at all*.

I wanted to get this up for review today, but I'd be rather amazed if it completely worked through the UI given that the entirety of the testing on the feature so far has been through the unit tests. However, test coverage is good enough in this code that I think the feature is close.