feat(bitbucket): Bitbucket Server platform support (#2774)

docs/self-hosting.md

lib/platform/bitbucket-server/README.md

lib/platform/bitbucket-server/bb-got-wrapper.js

lib/platform/bitbucket-server/index.js

lib/platform/bitbucket-server/utils.js

lib/platform/git/storage.js

lib/platform/index.js

lib/util/host-rules.js

lib/workers/global/index.js

test/_fixtures/bitbucket-server/responses.js

...latform/bitbucket-server/__snapshots__/index.spec.js.snap

test/platform/bitbucket-server/index.spec.js