Add maturity to categories

Add maturity, new_orleans_target_maturity, and 2019_target_maturity to categories.yml with possible values of none, minimal, usable, and lovable to identity what level we believe each category to currently be, and where we want them by the 2019 summit in New Orleans, and by the end of 2019.

The baseline rules are:

1. For new categories that will first ship in 2019, we expect categories to be minimal by the summit and usable by the end of the year.
2. For new categories for 2018 or earlier, we expect them to be usable by the summit.
3. For categories specifically identified, we aim to be lovable by the end of the year.

Not in this MR, I would like to take this data and visualize a maturity table.

Review app: https://msp-add-maturity.about.gitlab.com/handbook/product/categories/maturity/index.html

/cc @sytses @JobV