Label shepherd PRs so they're searchable

[parshap]

I can currently search for PRs opened via Shepherd with a GitHub search string like this: org:nerdwallet is:pr shepherd eslint, but that's somewhat fuzzy and it'd be good to be able to do a more precise search.

My suggestion is to support a search pattern like this: org:nerdwallet label:shepherd-migration shepherd_migration_2018_06_25_eslint_config_rename.

• Apply a shepherd-migration label to all PRs
• Generate a unique string that gets put in the body text of the PR

The unique string can be generated off of the "id" of the migration. This string should contain only underscores and not hyphens as GitHub search breaks up hyphenated words into individual words and does allow an exact match (regex for valid strings should be something like /[a-zA-Z0-9_]+/). I would do a string search and replace for anything that doesn't match that regex and replace it with an underscore.

The use case for this is to be able to use the GitHub UI to get a quick idea of what state a certain migration is in and help move PRs that are still open along.