Support enterprise level migrator role

[dylan-smith]

Description

The backend API now supports granting migrator role at the enterprise level (I believe it will iterate over all orgs and grant it on each org).

grant-migrator-role command should support --github-org or --github-enterprise flags to call the appropriate API.

Same applies for revoke-migrator-role

[timrogers]

I'm not sure the description above is correct, as the GrantMigratorRole mutation seems to only support an organization as a target. But we could nevertheless implement this!