Upload migration archives for GHES and Bitbucket direct to GitHub, rather than going via a blob storage provider

[timrogers]

Description

Migrations from GitHub Enterprise Server (GHES), Bitbucket Server and Bitbucket Data require the data being migrated (a "migration archive") to be uploaded to a blob storage provider, exposing a URL which github.com can use to download the data to continue the migration.

For GHES 3.8+, the GHES applicance itself handles the upload to a configured blob storage provider. For GHES <3.8 and Bitbucket, this CLI does the upload to blob storage.

For some customers, using a public cloud blob storage provider is difficult or possible.

We should switch to data being uploaded to GitHub instead, since the customer trusts GitHub.