Replace CODEOWNERS with CODENOTIFY

Created by: nicksnyder

Background

This PR replaces https://github.com/sourcegraph/sourcegraph/pull/11718.

For reasons mentioned in that other PR, I suspect that we would be better off not using CODEOWNERS.

The anonymous survey indicates that the dev team agrees: Source: https://docs.google.com/forms/d/e/1FAIpQLSfjDJxP1wdfEuTZdeNqTsCxfXD2HIYx_5aiTC17B6LyXwYorw/viewanalytics

The biggest concern was devs not being notified of changes that they care about.

This PR

This PR is the beginning of two related, but independent, experiments:

1. We are not going to use CODEOWNERS for the rest of September, and October.
2. I have created a tool called codenotify which aims to address the use case of allowing devs to be notified of changes that they care about (without creating the expectation of review). After this is merged, devs can experiment with adding CODENOTIFY files. Feedback on the behavior of codenotify is obviously welcome at any time.

At the end of October I will send out a survey to see how the team feels after living with these changes for a full release cycle.

Valid outcomes of this experiment include:

1. Everyone hates this and we go back to using only CODEOWNERS.
2. Devs find value in codenotify but want to use CODEOWNERS as well in some cases.
3. Devs are happy to only use codenotify and keep CODEOWNERS empty.

Given the data already collected in the previous survey, and my conviction that this is worth trying, I plan on merging this tomorrow.