Make GithubUser and GithubOrganisation less GitHub specific

[andrew]

To be able to pull in and store Users and Orgs from Bitbucket and Gitlab we need to change the GithubUser and GithubOrganisation classes into something like RepositoryUser and RepositoryOrganisation

note: GitLab calls their organisations "Groups" and Bitbucket calls their orgs "Teams"

Steps

• Rename GithubUser and GithubOrganisation classes
• Add host_type to GithubUser and GithubOrganisation tables
• Rename github specific fields in GithubUser and GithubOrganisation tables (with aliases for API responses)
• Deploy with migrations
• Prefill all existing user and org records host_type with GitHub
• Scope all user/org queries by host_type
• Extract github specific methods into a repository owner module