Add repository archiving policy

# 🚀 Feature Proposal

Unmaintained repositories should be archived. Ideally, people would do this but robots are much better at doing regularly-scheduled maintenance like that than people are. Let's:

1. Develop a policy for when unmaintained (i.e. un-updated) repositories get archived automatically
1. Implement a regularly-scheduled archive process for unmaintained repositories

## Motivation

We want to make it clear when repositories are no longer maintained and reduce confusion and expectations. Code rot sets in as soon as code is written.

Repositories can be easily un-archived through the Settings menu and it will prompt someone to maybe do something about that repo.

## Example

GSA 18F/TTS uses an [automated archiving process to archive after 90 days of inactivity](https://github.com/18F/ghad/tree/master) per their [open source policy section on archiving](https://github.com/18F/open-source-policy/blob/master/practice.md#archiving-a-repository).