Closes: #11755
Changes:
- Created a new model: `Platform` that has a single attribute:
`currentUnfrozenGitHubPrNumbers`
- Updated bootstrap.js to throw an error if more than one platform
record exists, and to create a platform record when the server is lifted
with the `--drop` flag.
- Updated the receive-from-github webhook to use a Platform record to
track the PRs that are currently unfrozen in the fleetdm/fleet repo.
Before this Pr is merged, we will need to:
- [x] Migrate the Fleet website's database to add the new database
table.
- [x] Create a single platform record.
