bin/pick-ui: Add a new maintainer script for picking patches

In the long term the goal of this script is to nearly completely automate the process of picking stable nominations, in a well tested way.

In the short term the goal is to provide a better, faster UI to interact with stable nominations.

This script offers a few basic improvements over the get-pick-list script

1. It saves history

• on each run it only never looks over the same master commit twice, this makes it much faster
• It is able to store denominated and backported changes in the same way

2. It provides a quick way to apply patches, simply pressing 'c' will attempt to commit a patch, and when that fails a pop-up is given explaining that it failed as well as as quick way to abort the commit
3. It can note things like a nomination that is reverted "Patch A: fixes" "some patches" "Revert patch A" and automatically mark both patches as unnecessary
4. I can find things like "Patch A fixes somthing picked" "some patches" "Patch B fixes Patch A" in one iteration, not so it doesn't need to be run multiple times like get-pick-list.sh
5. because the full history is checked into git if a maintainer switch is needed mid cycle the person taking over has all of the work done by the first maintainer
6. this comes with tests, meaning additional changes can be tested to fix regressions

I've already been using this on the 19.3 branch (as you may have noticed), and worked out quite a few bugs there already. right now I'm trying to get some feedback on anything else that needs to be fixed before this can be upstreamed.

Ah, this does depend on python 3.6+ and a curses-like toolkit called urwid, in case you want to run it yourself.