refactor: move github and gitlab api to platform (#1037)

lib/config/github-app.js

lib/config/index.js

lib/platform/gh-got-wrapper.js

lib/platform/github.js

lib/platform/gitlab.js

lib/platform/gl-got-wrapper.js

lib/workers/repository/apis.js

test/config/github-app.spec.js

test/platform/__snapshots__/github.spec.js.snap

test/platform/__snapshots__/gitlab.spec.js.snap

test/platform/gh-got-wrapper.spec.js

test/platform/github.spec.js

test/platform/gitlab.spec.js

test/platform/gl-got-wrapper.spec.js

test/workers/repository/apis.spec.js