Add support for personal metadata repos

right now pull-review is optimized for monorepos, where all the notification metadata lives in the same repo as the code. this means that it must be duplicated if pull-review is added to another repo in the organization, and the data can thus become stale.

one way around this is a personal notification metadata repo, like `imsky/pull-review-notification` with a single file inside it that specifies the Slack, etc. handles to use for the user (can also specify the Slack workspace).

No files, tests, or entry points are named. Start by reading how pull-review currently locates notification metadata across repositories, then determine the configuration and lookup flow needed for a personal metadata repository; done means the specified personal repository can provide notification handles for multiple organization repositories without duplicated metadata.

Written by the indexing model from the issue text.